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[Frontispiece 





Photo-micrographs of Grain of various Skins {A. Seymour- J ones). 

I. Cow-hide; 2. Calf-skin; 3. East India Goat ; 4. Pig-skin; 
5. East India Sheep; 6. Welsh Sheep. 



THE PRINCIPLES OF 
LEATHER MANUFACTURE 



Em.-Prof. H. R. PROCTER, D.Sc, F.I.C. 

LATE PROFESSOR OF LEATHER INDUSTRIES AT THE UNIVERSITY OF LEEDS ; 

HON. DIRECTOR OF THE PROCTER INTERNATIONAL RESEARCH LABORATORY; 

HON. MEMBER OF THE WORSHIPFUL COMPANY OF LEATHERSELLERS 



SECOND 'EDITION 



-=^ 




1Rew l^orh 

D. VAN NOSTRAND COMPANY 
8 ^^^ARREN STREET 

1922 






2)eMcateD to 

THE MEMORY OF 

THE LATE PROFESSOR F. L. KNAPP 

GEHEIMEN HOFRATH, DR. PHIL. AND DR. ING. 

THE PIONEER OF SCIENTIFIC RESEARCH 
IN LEATHER MANUFACTURE 



'^ "\'=\ ^ ^0 % 





PREFACE TO SECOND EDITION 

In the eighteen years which have elapsed since the pubhcation 
of the first edition, not onl}^ have great advances been made in 
the chemical knowledge on M^hich the manufacture is based, 
but new points of view have been gained, involving an acquaint- 
ance with physical and colloidal chemistry which is not yet 
uni\'ersal among leather-trade chemists, and I have felt com- 
pelled to enter into elementary details of these branches of science 
which, no doubt, in a few years will be among the commonplaces 
of chemical knowledge. I must therefore repeat the apology 
of the first preface, that to some of my readers much will neces- 
sarily seem too elementary, while to others it may prove too 
difficult, though I have endeavoured, as much as possible, to 
confine mathematical treatment to notes and appendices. It may 
be pointed out, ho\vever, that the advance of the future will 
necessarily tend to exact and mathematical methods, and that 
many theories which seem plausible enough when stated qualita- 
tively and in general terms will break down when submitted to 
the rigorous examination of mathematics. 

The third volume on the Methods of Leather Mamijacture, 
which was suggested in the former preface, yet remains to be 
written, but advancing age renders such a work impossible to 
the present writer, and its execution must be left to some of the 
young and well-trained chemists who are now devoting themselves 
to the trade. 

It is my pleasant duty to acknowledge much indebtedness to 
Mr F. C. Thompson, my Assistant in the Procter International 
Research Laboratory, not only for re-writing the chapters on the 
Chemistry of Hide and on that of the Tannins, with which he 
has made himself specially familiar, but for much general assist- 
ance both in the preparation of the work and in the reading of 
proofs ; to Mr W. R. Atkin for his permission to insert the paper 
on hydrion measurement which appears in Appendix D, and also 
for help in proof-reading ; and to the various gentlemen who 
have given information and allowed me to use their blocks and 
drawings in illustration ; and especially to Mr A. Seymour- Jones, 
Professor McCandhsh, Mr J. T. Wood, and Mr M. C. Lamb. 

The Procter International Research Laboratory, 
University of Leeds. 



PREFACE TO FIRST EDITION 

The origin of the present work was an attempt to prepare a 
second edition of the Httle Text-hook of Tanning which the 
Author pubhshed in 1885, and which has been long out of print. 
Though persevered in for years, the work was never brought 
to completion, partly owing to the constant pressure of other 
duties, but still more to the rapid advances which have been 
made in our knowledge of the subject, and in the scientific thought 
which has been devoted to it. For his share in the initiation 
of this work much credit is due to Wilhelm Eitner, Director of 
the Imperial Royal Research Institute for Leather Industries 
in Vienna, but the advance he began has been energetically 
carried forward not only in Vienna, but in the Tanning Schools 
and Research Institutes of Freiberg, Leeds, London, Liege, 
Copenhagen, Berlin, and elsewhere, and to a less extent in private 
laboratories. 

Under the pressure of this rapid growth, as it was impossible 
to complete the work as a whole, the Author published an instal- 
ment dealing with the purely chemical side of the subject in 1898, 
under the title of the Leather Industries Laboratory Book ; which 
has been translated into German, French, and Italian, and of 
which the English edition is rapidly approaching exhaustion. 

The present work, which should by right have preceded the 
Laboratory Book (and which frequently refers to it as 
" L.I.L.B."), attempts to deal with the general scientific prin- 
ciples of the industry, without describing in detail its practical 
methods (though incidentally many practical points are dis- 
cussed). To complete the subject a third volume ought to be 
written, giving working details of the various methods of manu- 
facture ; but apart from the difficulty of the subject, and the 
weariness of " making many books," the methods of trade are 
so fluctuating, and dependent on temporary conditions, that 
they have not the same permanent value as the record of scientific 
advance. 

As the present volume is intended to appeal both to the 
chemist and to the practical tanner, it must to a certain extent 
fail in both, since many matters are included which are already 
familiar to the former, and, it is to be feared, some which may 



PREFACE TO FIRST EDITION vii 

prove difficult to tlie latter. For these and other imperfections 
the Author claims the indulgence of his Readers. 

The Author must here acknowledge his indebtedness to Dr 
Tom Guthrie and to Mr A. B. Searle for assistance in writing 
several of the chapters ; to Dr A. Turnbull and Mr F. A. Blockey 
for much help in reading proofs and preparing the MS. for the 
press ; and to the many gentlemen who have furnished or allowed 
him to use their blocks and drawings in illustration. 

The Yorkshire College, 
Leeds. 



CONTENTS 



I. Introductory and Historical ... 
II. Preliminary Sketch of Leather Manufacture 

III. The Living Cell ..... 

IV. Putrefaction and Fermentation 
V. Antiseptics and Disinfectants 

VI. Origin and Curing of Hides and Skins . 
VII. Structure and Growth of Skin 
VIII. Water as used in the Tannery 
IX. Physical Chemistry of Leather Manufacture 
X. The Colloid State ..... 
XL The Chemistry of Hide .... 
XII. Soaking and Softening of Hides and Skins 

XIII. Depilation ...... 

XIV. Deliming, Bating, Puering, and Drenching 
XV Pickling and Depickling . . . 

XVI. Alum Tannage or Tawing 
XVII. Chrome and Iron Tannages 
XVIII. Vegetable Tanning Materials . . 

XIX. The Chemistry of Tannins 
XX. Sampling and Analysis of Tanning Materials 
XXI. Principles of the Vegetable Tanning Process 
XXII. Combination Tannages 

XXIII. The Grinding of Tanning Materials 

XXIV. Extraction and Extract-making 
XXV. Fats, Soaps, Oils, and Waxes . 

XXVI. Oil-tannages and Oils in Currying 
XXVII. Japanned and Enamelled Leathers 
XXVIII. Dyes and Dyeing . . . 

XXIX. Evaporation, Heating, and Drying . 
XXX. Construction and Maintenance of Tanneries 
XXXI. Waste Products and their Disposal 
XXXII. Conclusion ...... 



PAGE 

I 

7 

lO 

15 

21 

32 

48 

66 

84 

107 

125 

156 

166 

200 

234 
240 

254 

278 

337 
345 
355 
375 
380 

392 

425- 

457' 

475 

487 

512 

538 

555 

572 



Appendices : — ' 

A. The Metrical System ...... 580 

B. The Theory of SweUing of Gelatine and Hide (Procter 

and Wilson) ....... 582 

C. List of Coal-tar Dyes now available for Dyeing and 

Staining Leather (M. C. Lamb) .... 640 

D. Acidity of Tan Liquors (W. R. Atkin and F. C. 

Thompson) . . . . . . . 656 

E. Caustic Alkalinity of Lime Liquors (W. R. and J. Atkin) 667 



THE PRINCIPLES OF 
LEATHER MANUFACTURE 

CHAPTER I 

INTRODUCTORY AND HISTORICAL 

The origin of leather manufacture dates far back in the pre- 
historic ages, and was probably one of the earliest arts practised 
by mankind. The relics which have come down to us from 
palaeolithic times, and the experience of the modern explorer, alike 
tell us that agriculture is a later and a higher stage of develop- 
ment than the life of the hunter ; and since, in the colder regions, 
clothing of some kind must always have been a necessity, we may 
conclude that it was first furnished by the skins of animals.^ 

While wet skins putrefy and decay, dry ones are hard and 
horny ; and nothing could be more natural to the hunter than 
to try to remedy this by rubbing the drying skin with the fat of 
the animal, of which he must have noticed the softening effect on 
his own skin. By this means a soft and durable leather may be 
produced, and this process of rubbing and kneading with greasy 
and albuminous matters, such as fat, brains, milk, butter, and 
egg-yolks, is in use to this day, alike by the Tartars on Asiatic 
steppes and the Indians on American prairies ; and not only so, 
but we ourselves still use the same principle in the dressing of 
our finest furs, and in the manufacture of chamois, and many 
sorts of lace- and belt-leathers. 

Such a process is described in the Iliad (xvii. 389-393) in the 
account of the struggle over the body of Patroclus : 

" As when a man 
A huge ox-hide drunken with slippery lard 
Gives to be stretched, his servants all around 
Disposed, just intervals between, the task 
Ply strenuous, and while many straining hard 
Extend it equal on all sides, it sweats 
The moisture out and drinks the unction in." 

^ See also Gen. iii. 21. 



2 PRINCIPLES OF LEATHER MANUFACTURE 

It must also have been early noticed that wood smoke, which 
in those days was inseparable from the use of fire, had an anti- 
septic and preservative effect on skins which were dried in it, 
and smoked leathers are still made in America, both by the 
Indians and by more civilised leather manufacturers. To this 
method the Psalmist refers ^ when he says, " I am become like a 
bottle in the smoke " ; and such bottles, made of the entire skin 
of the goat, are still familiar to travellers in the East. 

The use of vegetable tanning materials, though prehistoric, 
is probably less ancient than the methods I have described, and 
may possibly have been discovered in early attempts at dyeing ; 
an art which perhaps had its origin even before the use of clothing ! 
The tannins are very widely distributed in the vegetable kingdom, 
and most barks, and many fruits, are capable of making leather. 

The employment of alum and salt in tanning was probably 
of still later introduction, and must have originated in countries 
where alum is found as a natural product. The art was lost or 
unknown in Europe till introduced into Spain by the Moors. 

Leather manufacture reached considerable perfection in ancient 
Egypt. A granite carving, probably at least 4000 years old, is 
preserved in the Berlin Museum, in which leather-dressers are 
represented. One is taking a tiger-skin from a tub or pit, a 
second is employed at another tub, while a third is working a 
skin upon a table. Embossed and gilt leather straps have been 
found on a mummy of the ninth century B.C., and an Egyptian 
boat-cover of embossed goat leather, as well as shoes of dyed 
and painted morocco, are still in comparatively good preserva- 
tion. The art is of very early date in China, and was well under- 
stood by the Greeks and Romans. In the Grosvenor Museum 
at Chester is the sole of a Roman caliga, studded with bronze 
nails, which is yet pretty flexible ; and Roman shoes have been 
found in other parts of England in not much worse condition 
than those which tramps leave by the wayside. After the fall 
of the Roman empire many arts were lost to Europe, and it was 
not until the Moorish invasion of Spain that the art of dyeing 
and finishing the finer kinds of leather was reintroduced. 

England was very backward in this manufacture up to the 
end of the eighteenth century, owing to the fossilising influence 
of much paternal legislation, and of certain excise-duties, which 
were only repealed in 1830. Since this time the art has made 
rapid strides, especially in the use of labour-saving machinery, 
and England may at the present moment be considered fairly 
abreast of any other country as a whole ; though in some special 

^ Ps. cxix. 83. 



INTRODUCTORY AND HISTORICAL 3 

manufactures we are surpassed by the Continent and by America. 
In making comparisons of this kind, it must, however, be remem- 
bered that, especially in sole-leather tannage, the most rapid 
progress has been made during the last few years in those countries 
which were more backward, and that therefore our superiority 
is much less pronounced than formerly, and in a few years may 
cease to exist unless marked improvements are introduced in 
the methods of production. The past twenty years have, how- 
ever, been marked by very considerable advance, and the applica- 
tion of science to the manufacture has been very noticeable. 

In the sketch of the development of leather manufacture 
which has just been given, it has been implied that its object is 
to convert the putrescible animal skin into a material which is 
permanent and not readily subject to decay, while retaining 
sufficient softness or flexibility for the purposes for which it is 
intended. As these range from boot-soles to kid-gloves, there 
are wide divergences, not only in the processes employed, but 
also in the materials used and in the principles of their application. 
The most important method of producing leather is still by 
the use of vegetable tanning materials, and this is perhaps the 
only one which is really entitled to be called " tanning," though 
the distinction is not very strictly adhered to. It includes the 
whole range — from sole leather, through strap, harness and 
dressing leather, to calf and goat skins, and the various sumach 
tannages which jdeld morocco and its imitations. All of these 
products but the first and the !ast undergo, after tanning, the 
further processes of "currying," of which the most important 
operation consists in " stuffing " with oily and fatty matters, 
both to increase the flexibility and to confer a certain amount 
of resistance to water. Sumach-tanned skins are not strictly 
" curried," but usually receive a certain amount of oil in the 
process of " finishing." 

With the vegetable tans we may include Stiasny's synthetic 
organic tannins, " Neradol D " and others, produced by con- 
densation of formaldehyde with phenolsulphonic acids (see also 
P- 343) > which produce a perfectly white leather when used 
alone, and can be combined with other tans to give a paler 
leather and to quicken tannage. 

Next in importance to the vegetable tannages are those pro- 
duced by salts of chromium and aluminium, including all the 
various " chrome " leathers, and the " tawed " or " white " 
leathers, used for whip-lashes, belt -laces, and aprons, as well as 
for calf- and glove-kid. 

Connecting links between these and the vegetable tannages 



4 PRINCIPLES OF LEATHER MANUFACTURE 

are found in " green leather," " Dongola " and other combination 
tannages in which alum and salt are employed in conjunction with 
vegetable materials ; and in " semi-chrome " and other leathers 
in which chrome salts are used instead of those of alumina. 
Formaldehyde also is becoming of importance as an adjunct to 
other tannages. 

In the production of calf- and glove-kid, in addition to alum 
and salt, albuminous and fatty matters, such as egg-yolk, olive 
oil, and the gluten of flour, play a considerable part, and are thus 
linked both to the primitive methods in use by the Indians and 
Kalmucks, and to those by which "crown" and "Helvetia" 
leather, and many other forms of belt- and lace-leathers, are now 
produced by treatment with fats and albumens. 

From these again the step is a short one to the " chamois " 
and "buff" leathers, and the German "fettgar" leathers, in 
which oils and fats only are used. 

In an attempt to view all these complex processes from the 
scientific standpoint, the reader should constantly realise that 
the present methods of leather manufacture are the results of 
tens of centuries of experience, and of innumerable forgotten 
failures, and must not therefore expect that they can be easily 
superseded. Science must follow before it can lead, and its first 
duty is to try to understand the reasons and principles of our 
present practice, for we can only build the new on the foundation 
of what has been already learned. Another fact, which is 
scarcely understood by the practical man in his demands on 
science, is that in leather manufacture every question which is 
raised seems to rest on the most recondite problems of chemistry 
and physics ; the chemistry of some of the most complex of 
organic compounds, and the physics of solution, of osmose, and 
of the structure of colloid bodies — problems which are yet far 
from completely conquered by the highest science of the day. 
Many of these problems are, however,. rapidly yielding to investi- 
gation, and we may anticipate that science, and especially 
chemistry, will pl^y a constantly increasing part in the technology 
of leather manufacture. 

It may seem bold to attempt the scientific treatment of such 
a subject at all ; and, indeed, it must be admitted that our 
knowledge is still inadequate for its complete accomplishment, 
but much has been done in the last twenty years, and this can 
at least be summarised and arranged in an available form. The 
subject falls naturally into two sections, in the first of which the 
processes of manufacture would only be described in general 
terms, and with sufficient fulness to enable the reader to under- 



INTRODUCTORY AND HISTORICAL 5 

stand the scientific considerations on which they are based, and 
the methods of investigation which can be apphed to them ; 
while in the second an effort should be made to give working 
details of the various processes sufficient to enable those with a 
general knowledge of the trade to experiment successfully in its 
various branches. It was at first intended that these two sections 
should be published in one book as a second edition to the Author's 
Text-hook of Tanning, but owing to the long delay in its publication, 
it was decided to publish the first section under the present title 
The Principles of Leather Manufacture, leaving the latter section 
Processes of Leather Manufacture to a later date. It is improbable 
that, for the present writer, this date will ever arrive, but the 
work has been to some extent done by others, and especially by 
Bennett in his Manufacture of Leather (Constable & Co., London, 
1909) ; while the more strictly chemical portion has already 
appeared in the Leather Industries Laboratory Book, frequently 
referred to in the following pages under the abbreviation 
" L.I.L.B.," 2,ndinih.e Leather Chemists' Pocket Book, " L.C.P.B." 
Where quantities and details are given, they must not be taken as 
recipes to be blindly followed ; or even, in every case, as the best- 
known methods ; but rather as mere guides to experiment, which 
must be modified to suit varying conditions and requirements. It 
is the special virtue of the scientific, as opposed to the merely 
traditional, way of looking at such questions, that knowing the 
cause and effect of each part of the process, it can so adjust 
them as to get over difficulties, and to suit novel conditions ; 
although much time may be wasted in useless experiments, if 
approximate and practical quantities and methods are not known. 
It is needless to add that many methods are jealously preserved 
as trade secrets, and full details are frequently unattainable, 
though this is less the case than formerly. 

After what has just been said, it may be well to emphasise 
the great importance of practical knowledge and experience to 
the leather manufacturer. Even in trades which have reached 
the highest scientific development, such, for instance, as the 
manufacture of the coal-tar colours, the small experiments of the 
laboratory are not transformed into manufacturing operations 
without experience and sometimes even failure ; and this must 
still more often be the case in a trade like that of leather-making, 
where our knowledge of the actual changes involved is so in- 
complete. On the other hand, the cost of experiments on a 
manufacturing scale is usually so heavy that the least scientific 
must admit the advantage of learning all which the laboratory 
can teach before venturing further ; while even our present 



6 PRINCIPLES OF LEATHER MANUFACTURE 

imperfect knowledge of the chemical changes involved will often 
warn us off hopeless experiments, and give us hints of the direc- 
tions in which success may be attained. A knowledge of 
chemistry will certainly prove at least as important to the future 
of our trade as that of mechanics has been in the past. 



CHAPTER II 

INTRODUCTORY SKETCH OF LEATHER MANUFACTURE 

The object of tanning has been stated to be the rendering of 
animal skin imputrescible and pliable, but as we now rarely 
require leather with the hair on, preliminary processes are needed 
to remove it and to fit the skin for tanning ; and the nature of 
these processes has great influence on the subsequent character 
of the leather produced. 

The first step is usually a washing of the skin to remove blood 
and dirt ; while, where it has been salted or dried, a more thorough 
soaking is needed to remove the salt, and to restore the skin to 
its original soft and permeable condition. 

The hair is then loosened by softening and partial solution 
of the epidermis structures (see p. 49) in which it is rooted. 
This is most generally accomplished by soaking for some days in 
milk of lime, which is usually assisted by the addition of alkaline 
sulphides. When the latter are used in concentrated solution, 
the hair itself, as well as the epidermis tissues, is softened and 
destroyed in the course of a few hours. The lime not only serves 
to loosen the hair, but swells and splits up the fibre-bundles of 
which the hide tissue is composed, and so fits it to receive the 
tannage {cp. p. 58) ; and the process is always complicated and 
assisted by bacterial activity. 

For some purposes a regulated putrefactive process is indeed 
substituted for the liming ; the hides or skins being hung in a 
moist and warm chamber (see p. 166), when the soft mucous layer 
which forms the inner part of the epidermis is disintegrated, 
partly by direct putrefaction, partly by the action of the ammonia 
evolved, so that the hair can be scraped off. In this case the 
hide-fibre is not swollen, and the necessary swelhng has to be 
obtained by subsequent processes. 

In whatever way the hair has been loosened, it is either scraped 
off with a blunt and somewhat curved two-handled knife on a 
sloping rounded " beam " of wood or metal, or removed by a 
suitable machine, this operation being termed " unhairing " 
(see p. 191). 

This is generally followed by " fleshing," which is performed 
on the same beam with a somewhat similar knife, which, how- 



8 PRINCIPLES OF LEATHER MANUFACTURE 

ever, is two-edged and sharp. In this operation, portions of 
flesh, and the fat and loose tissue which underhe the true skin 
(see p. 193), are removed by scraping and cutting. Machines for 
fleshing are also largely in use (see p. 196). 

For sole leather, the hide, after some washing in soft water or 
treatment with weak acid solutions to cleanse from lime, is then 
ready for the actual tanning process ; but for the softer leathers 
more thorough treatment is needed to remove the lime, and to 
still further soften the skin by solution and removal of a portion 
of the cementing substance and of the elastin fibres. 

This treatment was generally of a fermentative or putrefactive 
nature, and the most common form was that known as " bating," 
which consists in steeping in a fermenting infusion of pigeon- or 
hen-dung. The theory of its action is not yet thoroughly under- 
stood, but the effect is largely due to the unorganised hydro- 
lysing ferments produced by the bacteria present ; while at 
the same time the lime is neutralised and removed by the weak 
organic acids and salts of ammonia which are produced, and 
the fibres, which had been plump and swollen with lime, becomes 
extremely relaxed and flaccid. This process and that of puering 
have been largely superseded by the direct use of trypstic fer- 
ments derived from the pancreas of animals in conjunction with 
ammonium chloride to remove the lime (Chap. XIV.) . 

In the lightest leathers, such as kid- and lamb-skins for gloves, 
and goat and sheep for moroccos and the like, dog-dung is sub- 
stituted for that of fowls, and the process is then called " puering " 
(see p. 230). 

These processes are often followed by " drenching," which 
sometimes indeed takes their place, the skins being soaked in a 
fermenting bran infusion. In this, the small quantities of acetic 
and lactic acid formed by fermentation are the active agents, 
neutralising and dissolving the lime, and cleansing and slightly 
plumping the pelt (see p. 214). 

In recent times these unpleasant processes have been largely 
supplemented, and in some cases superseded, by purely chemical- 
means, which are safer and more economical. 

The tanning process which follows consists in soaking the 
pelt in infusions of various vegetable products containing bodies 
of the class known as " tannins," which have the power of com- 
bining with skin-fibre and converting it into leather. 

If at first strong infusions were used, they would act too 
violently on the surface of the skin, hardening and contracting it 
so that the subsequent tannage of the interior would be impeded, 
and the " grain " or outer surface would be " drawn " and 



SKETCH OF LEATHER MANUFACTURE 9 

wrinkled. This is avoided by the use at first of very weak in- 
fusions which have already been partially exhausted on goods 
in a more advanced stage. In the later part of the process much 
stronger solutions are employed, and the hides are frequently 
" dusted " in them with ground tanning material. 

In the case of sole leather, these processes may require from 
two to twelve months for completion ; after which the leather is 
dried, smoothed, and compressed by mechanical means, and is 
then ready for use. The time of tannage is now often shortened 
by " drumming " and other processes of agitation. 

Dressing-leathers, ranging from calf-skins to harness-hides, 
rec-eive a much shorter tannage, and a subsequent treatment with 
fats and oils, which, together with mechanical manipulations, 
constitute " currying." The thin film of grease distributed over 
the surface of the fibres renders them supple, and to some extent 
waterproof. 

The lighter fancy leathers, such as morocco, are dyed, and 
undergo many complex processes to fit them for their required 
purposes and improve their appearance. 

Many skins, such as calf, glove, and glace kid, are not tanned, 
but " tawed " by a solution of alum and salt, which is often sup- 
plemented with mixtures of flour and egg-yolk to fill and soften 
the leather. 

Salts of chromium are now largely employed in place of alum 
and salt, and produce an equally soft, but more permanent 
and enduring, leather, and the process is also applied to the 
production of the firmer leathers required for soles and beltings. 

Lastly, wash-leather, or so-called " chamois," and buff -leather 
are produced by fulling the prepared pelt with fish or whale oil, 
which converts the skin into leather by subsequent oxidation, 
during which aldehydes are evolved. 



CHAPTER III 

THE LIVING CELL 

The larger part of the materials employed in leather manufac- 
ture are organic in their origin, and the skin itself is an organised 
structure, while the life-processes of putrefaction and fermentation 
play a large part in the tannery. Some knowledge, therefore, 
of biological structures and processes is necessary to a full under- 
standing of much which follows, and a few words are not out of 
place with regard to the foundations of life itself. 

The bricks of which all living structures are built are the 
living " cells " and their products, and these first elements differ 
little, if at all, whethei the life is animal or vegetable, the dis- 
tinction being produced rather by the way in which they are put 
together than by differences in the cells themselves. This is so 
much the case that it is often difficult to decide in which of the 
two classes to place the simplest organisms, since many of these 
forms are capable of active movement, and their modes of 
nutrition and reproduction are common to both kingdoms. 

In its simplest form, the cell, whether animal or vegetable, 
is strictly speaking not a cell at all, but consists merely of a 
minute mass of living jelly or protoplasm. Such is the amoeba 
found in water and damp soil, such are the lymph-cells and 
White blood-corpuscles of our bodies, and such also some stages 
at least of the lowest forms of fungi, like the Mthalmm septicum, 
which is sometimes found on old tan-heaps as a crawling mass 
of yellow slime. If a drop of saliva be examined with the micro- 
scope under a cover-glass, with one-sixth objective and small 
opening of diaphragm, ^ a few scattered semi-transparent objects 
will be found, of the apparent size of a lentil or small pea, and 
of rounded form. These are lymph-corpuscles (fig. i). Their 
contents are fuU of small granules, and if they be observed quickly, 
or if the slide be kept at about the warmth of the body, it will 
be noticed that these are in constant streaming motion. If the 
warmth can be kept constant, which is difficult without special 
apparatus, and the cells can be observed from time to time, it 
may be seen that they lose their circular form, and put out pro- 

^ For details of microscopic manipulation in this and the following 
chapter see L.I.L.B., pp. 411 et seq., and L.C.P.B., pp. 199 et seq. 

.10 



THE LIVING CELL ii 

tuberances (pseudopodia, "false feet "), one of which will gradually 
increase in bulk, till it absorbs the whole cell, which thus crawls 
about. It will now readily be understood how these cells wander 
through all the tissues of the body, passing through the smallest 
pores Hke the fairy who put her finger through a keyhole and 
grew on the other side till she was all through ! This independent 
vitality, in a warm and suitable nutrient liquid, may continue 
quite indefinitely. 





7jn.. dTH, 








Fig. I. — Lymph-corpuscle of frog, showing gradual change of form. 

(Ranvier.) 

It is possible that by close attention, a rounded or elongated 
body, somewhat like an oil-globule, may be seen within the cell, 
though it is generally more obvious when the latter has been 
killed and stained with a weak solution of iodine. This is the 
nucleus, and within it is a still smaller speck called the nucleolus, 
which bears an important, and as yet little understood, part in 
the life-history of the cell. After a period it undergoes certain 
compHcated changes in its internal structure, and divides into two, 
the nucleus elongates, and also divides, each half carrying with it 
a portion of the living protoplasmic jelly, and thus forming two 
complete and independent cells. This is the life-history, not only 
of the lymph-cell, but, with more or less modification, of every 
living cell or tissue. 



12 PRINCIPLES OF LEATHER MANUFACTURE 



These cells, like all living things, feed on the nutriment which 
surrounds them, and even enclose small particles of solid food, 
which are gradually dissolved and disappear. In this way the 
white blood-corpuscles are said to feed upon and destroy the still 
smaller organisms which gain access to the blood, and which 
might otherwise cause disease, and to absorb and remove dead 
tissues. The matter which cells consume is not, of course, de- 
stroyed, but simply converted into other forms, some of which 
are useless, or even poisonous to the cells, and which, like the 




Fig. 2. — Yeast-cells, much magnified. 

secretions of higher animals, are discharged into the surrounding 
fluids ; while others are retained, and contribute to the growth 
of the cell. Thus most vegetable cells secrete cellulose, or plant- 
tissue, which forms a wall enclosing the protoplasm, and so 
justifies the name of cell. If to warm water and a little sugar 
we add enough yeast to render it slightly milky, and examine 
it like the saliva, we shall have before us typical vegetable cells 
of the simplest form (fig. 2). There is the same granular proto- 
plasm, and there is the nucleus, though it cannot be seen without 
special preparation, the rounded spaces which look like nuclei 
being simply filled with transparent fluid, and called vacuoles. 
There is, however, no motion, as in the case of amoeba, for the 
cells are enclosed in a tough skin of cellulose, which will be evident 
if they are crushed by putting some folds of blotting-paper on the 
cover-glass and pressing it with the handle of a needle or a 
rounded glass rod, when the protoplasm will be forced out and 



THE LIVING CELL 



13 




the skin remain like a burst bladder. This will be more obvious 
if the cells are previously treated with iodine or magenta, which 
will stain the protoplasm, but not the membrane. It is easy to 
observe the multiplication of 
the yeast -cells, which is some- 
what different to that of the 
corpuscles. Instead of enlarg- 
ing as a whole, and dividing 
into two equal cells, a small 
bud appears on the side of the 
parent -cell, and enlarges till it 
becomes itself a parent-cell 

with buds of its own. These „ ^2 u.\. v ^ ^^ i-d ■ \ 

Fig. 3. — Epithelium-cells. (Ranvier.) 

do not break away at once, ^, pressure-marks ; ^, granular 

and hence chains and groups protoplasm. 

of attached cells are formed 

which are easily noticed in growing yeast if a microscope be 
employed. The principal nutriment of yeast is grape-sugar or 
glucose ; and much more of this is consumed than is needed to 

produce the cellulose wall and the sub- 
stance of new cells ; just as in the 
animal, sugar, starch, and fat are con- 
sumed to give heat and energy. In 
the yeast, this extra sugar is split up 
into carbon dioxide, which escapes as 
gas, and to which yeast owes its 
power of raising bread ; and into 
alcohol, which in too large proportion 
is poisonous to the yeast itself. 

In examining the saliva for lymph- 
cells, it is probable that some much 
larger objects may have been noticed 
of irregular polygonal outline and 
with a well-marked nucleus. These 
are cells from the lining epithelium of 
the mouth, and only^ differ from those 
of the epidermis of skin in their form 
and size (fig. 3). Note the markings 
Fig. 4. — Penicillium glaucum, caused by the pressure of overlapping 
a common green mould. cells. In these cells the wall is formed 
of keratin or horny tissue, which 
takes the place of the cellulose of the yeast. 

Other simple forms of cell are those of Saccharomyces myco- 
derma or tonda, which forms a skin on the surface of old liquors, 




14 PRINCIPLES OF LEATHER MANUFACTURE 

and which much resembles a small yeast ; and of the various 
ferments which are found in liquors, bates and drenches, which 
will be more fully described in the chapter following. 

Many of these, such as the acetic and lactic ferments, which, 
like all other bacteria, multiply by division, do not separate, but 
remain connected in chains or chaplets, like a string of beads. 
From these, the step is not a long one to the hyphce or stems of 
the higher moulds, which are too frequently found on leather 
which has been slowly dried, and which consist simply of tubular 
cells which elongate and divide by the formation of septa or cross- 
partitions, and thus build up a complicated plant-structure (fig. 
4). As we proceed higher in the scale of plant and animal life 
the forms and products of the cells become more varied, and 
instead of one single cell, fulfilling all the functions of the plant 
or animal, each class of cell has its own peculiar duties and 
properties, while all work together for the maintenance of the 
complex structure of which they form a part. 



CHAPTER IV 

PUTREFACTION AND FERMENTATION 

The chemical changes produced by the unicellular plants, such 
as yeasts and bacteria, to which allusion has been made in the 
last chapter, are known as fermentation and putrefaction, and are 
of such importance to the tanner, both for good and evil, that the 
subject must be treated in some detail. No scientific distinction 
exists between fermentation and putrefaction, though it is 
customary to restrict the latter term to those decompositions of 
nitrogenous animal matter which yield products of disagreeable 
smell and taste. 

The organisms which are the cause of both fermentation and 
putrefaction are known by the general term of " ferments." This 
term has also been extended in recent years so as to include the 
so-called " unorganised ferments " (enzymes, zymases), which are 
active digestive products secreted by bacteria and other living 
organisms, and which are constantly increasing in both scientific 
and technical importance. 

The organised ferments are again divided into three classes : — 

1. Moulds. 

2. Yeasts (Saccharomycetes) . 

3. Bacteria. 

The members of one class are distinguished from those of 
another by their form, and, more especially, by the substances 
they produce during their life-history. All three classes are 
now considered to be fungi. 

All enzymes possess the following three properties : — 

ST' 

1. They are nitrogenous bodies. 

2. They are unstable, i.e. they are destroyed by heat, 

chemicals, etc. 

3. A relatively small quantity of the ferment is capable of 

producing great changes in the substances upon which 
it acts, especially if the products of the change can be 
removed as they are formed. In these qualities they 
resemble the substances known as " catalysts " in 
chemistry, and their action must be regarded as a 
species of catalysis. 

15 



i6 PRINCIPLES OF LEATHER MANUFACTURE 

The general character of fermentation will be best understood 
by a closer study of the yeast-cell, which has already been de- 
scribed (p. 12), and its life-history briefly sketched. It has been 
shown that it is a growing plant of a very simple type, belonging 
to the fungi. These are devoid of the green colouring mattei 
which enables the higher plants to utilise the energy of sunlight 
in assimilating the carbonic acid of the atmosphere, exhaling its 
oxygen, and employing its carbon for the building up of tissue ; 
and they must therefore, like animals, have their nutriment ready 
formed, and capable of supplying energy by its oxidation. For 
yeast, as has been stated, the appropriate nourishment is glucose, 
or " grape-sugar." This is broken down, in the main, into the 
simpler compounds, alcohol and carbonic acid, while a small 
portion is utilised for the building up of the cell and the forma- 
tion of secondary products. The main reaction is represented 
by the following equation : — 

CeHiaOe = 2C2HeO + 2CO2 

Glucose Alcohol Carbon dioxide 

Yeast cannot directly ferment ordinary cane-sugar (C12H22O1]), 
but secretes a substance called invertase, which so acts on the 
sugar as to break it up, with absorption of one molecule of water, 
into two molecules of fermentable glucose (dextrose and levu- 
lose) which serve as nourishment for the yeast. -"^ This invertase 
is a type of the series of bodies which we have spoken of as 
" unorganised ferments," enzymes, or zymases, differing from the 
organised ferments in being simply chemical products without 
life or power of reproduction, but capable of breaking up an 
unlimited qua,ntity of the bodies on which they act, without 
themselves suffering change. The way in which this is done is 
not clearly understood, but some parallel may be found to it in 
the action of sulphuric acid on alcohol, of which it will convert 
an unlimited quantity into ether, without itself suffering any 
permanent change. The action of enzymes is limited to breaking 
down complex bodies into simpler forms, often with absorption 
of water, as in the case of sugar, while some of the products of 
living ferments are often complex, a part of their nutriment 
being broken down into simple products, such as carbonic acid, 
marsh gas, and ammonia, to supply the necessary energy to 
elaborate the remainder. 

Very many different unorganised ferments are known to exist, 
as they are not only produced by yeasts and bacteria, but are 

^ Compare O'Sullivan and Thompson, Jouvn. Chem. Soc, iSgo, p. 834 ; 
1891, p. 46. 



PUTREFACTION AND FERMENTATION 17 

formed by the cells of higher plants and animals ; thus the 
digestive principles, pepsin, trypsin, ptyalin, are of this character 
— ptyalin, like diastase, converting starch into sugar ; and such 
bodies fulfil many functions both in animal and vegetable 
economy. In fermentation, as in disease, it is often difficult to 
distinguish what is due to the direct action of bacteria and what 
to the unorganised ferments which they produce, and the question 
is further comphcated by the fact that in most natural fermenta- 
tions more than one ferment -organism is present. Sometimes 
the action of the unorganised ferments may be distinguished 
by the fact that the addition of chloroform has little effect on 
their activity, while it paralyses that of the living organism. By 
exposure to high temperature both are destroyed, the bacteria, 
yeasts, and moulds being killed and the unorganised ferments 
coagulated Hke white of egg, and so rendered inoperative. Many 
antiseptics also destroy the activity of both organisms and 
enzymes ; but others, like chloroform, have no action on the 
latter. In many cases, as in that of invertase, the actual zymase 
can be precipitated by alcohol from its aqueous solution, filtered 
off, and restored to activity by transference into water. Wood 
found that the enzjmies so separated from a puer liquor were 
still active after fourteen years. Since both classes of ferments 
are destroyed by high temperatures, all fermentation-processes 
are completely and permanently arrested by exposure to suffi- 
cient heat, and subsequent preservation in vessels so closed that 
no new ferment-germs can gain access. A familiar instance is 
that of tinned meats. All fully developed bacteria are destroyed 
by a very short exposure to a boiling temperature, and most 
by 60° to 70° C, but many species produce spores which are 
extremely difficult to destroy. The thermophilic bacteria 
discovered by Globig and further investigated by Rabinowitsch ^ 
thrive at a temperature of 60° C. About eight species of these 
are known, and they take part in the heating of hay and similar 
fermentations where high temperatures are involved, and are 
therefore presumably present in spent tan. 

The spores of the anthrax bacterium, the cause of the maUgnant 
pustule and of " wool-sorters' disease," are extremely resistant 
both to heat and to disinfectants. {Cp. p. 236.) 

For absolute sterilisation it is therefore necessary either to boil 
under pressure so as to raise the temperature to, say, 110° C, 
or to heat repeatedly for a short time to temperatures of 80° to 
100° C. at successive intervals of twenty-four hours, in order to 
allow the spores to develop. This process is frequently per- 
1 Centr. Blatt fur Bakt., II. Abth., vol. i. p. 585. 



i8 PRINCIPLES OF LEATHER MANUFACTURE 

formed for bacteriological observation in flasks or test-tubes 
merely stopped with a plug of sterilised cotton- wool, which has 
been found to filter efficiently the germs from the air which enters 
through it (see L.I.L.B., p. 440 ; L.C.P.B., pp. 209 et seq.). 

The ferment -organisms cannot thrive and multiply unless 
they have proper nourishment and conditions of growth, the 
amount of moisture and the temperature being two of the most 
important of the latter. Use is made of this in the preservation 
of many articles of food, etc., since by ensuring that at least one of 
the conditions necessary for growth shall be absent, these sub- 
stances are prevented from decomposing. For instance, hides 
are preserved by drying them ; the absence of sufficient moisture 
hindering the growth of any organisms in them so long as they 
are dry, but as soon as they become somewhat damp, putrefaction 
commences again. Similarly, foods are preserved by chilling or 
freezing. 

The waste products of organisms are often poisonous to them- 
selves, and for this reason fermentations frequently come to an 
end before the whole of the substance is fermented. Thus 
neither beer nor vinegar can be obtained of more than a certain 
.strength by direct fermentation, the alcohol or acetic acid check- 
ing the growth of their respective ferments. A solution of glucose 
" set " with the lactic ferment of sour milk will only produce 
lactic acid to the concentration of about one-half per cent. ; but 
if chalk be added, the lactic acid will be neutralised as produced, 
and the fermentation will go on till the whole of the glucose 
is converted into insoluble calcium lactate. ^ When this is accom- 
plished the lactic ferment dies from want of nutriment, and its 
place is taken by another organism, of which some germs are sure 
to be present, which ferments the calcium lactate into calcium 
butyrate. If the nourishment fails, or the conditions become 
less favourable for one ferment than for some other which exists 
even in small quantity in a liquid, the former is quickly over- 
grown and killed, and the latter takes its place. Thus the 
ordinary ferment of the bran drench will die out rapidly unless 
constantly transferred to fresh bran infusions. 

Many of the products of bacteria (like those of some of the 
higher plants) are intensely poisonous both to animals and 
man. Many of the severe symptoms of disease are caused 
by these poisons produced in the body. Thus the tetanus- 
bacteria produce a poison similar in its effects to strychnine, 

^ For the practical preparation of lactic acid the solution may contain 
7^ to II per cent, of glucose and some nitrogenous nourishment. The 
solution should be slightly acid. See Jotirii. Soc. Chem. Iiid., 1897, p. 516. 



PUTREFACTION AND FERMENTATION 19 

but even more virulent. Not only are such poisons produced by 
disease-bacteria in the body, but frequently also in the earlier 
stages of putrefactive fermentation. The latter are known as 
ptomaines, and when present in cheese and preserved foods 
are liable to cause poisoning. Such putrefactions are often 
unaccompanied by any disagreeable odour or flavour. 

The fermentations which are most important in the tannery 
are, firstly, the ordinary putrefaction which attacks hides as well 
as other animal matter, and which is usually a complicated 
process carried on by many sorts of bacteria and other micro- 
organisms. This may be regarded as generally injurious to 
the tanner ; but it is utilised for depilation in the " sweating " 
process and in the " staling " of sheep-skins, in both of which 
advantage is taken of the fact that the soft mucous layer of the 
epidermis, which contains the hair-roots, putrefies more rapidly 
than the fibrous structure of the hide itself. In soaking also, use 
is sometimes made of the power of putrefactive ferments to 
dissolve the cementing substance of the hide, though in this case 
with doubtful advantage to the tanner. In the liming process 
putrefaction makes itself felt when the limes are allowed to become 
stale and charged with animal matter, softening the hide and 
finally rendering the leather loose, empty, and inclined to " pipe," 
but in normal liming bacterial action plays an important part. 
Here the effect is useful if not carried too far. 

In bating and puering the action is almost entirely due to 
the enzymes and other products of bacterial activity, the original 
chemical constituents of the dung being apparently of minor 
importance. Naturally the liquid is adapted to the growth of 
many other organisms beside those acting most advantageously 
on the hide, and injury in the bates from wrong forms of putre- 
faction is very common, if indeed it is not always present in 
greater or less degree. 

In drenching the effect is, at first, entirely due to the weak 
acids produced by bacterial fermentation of the bran, but 
becomes complicated in its later stages by putrefactive and other 
fermentations, which may be desirable or otherwise. 

In the tanning liquors fermentation is not so marked, but is 
of great importance owing to the production of acids by bacterial 
action from the sugars present in the material. The acids them- 
selves are apt to be fermented and destroyed, principally by the 
oxidising action of Saccharomyces mycoderma and the higher 
moulds, which also act very destructively on the tannins. 

The effect of these acids on the hides is to swell them and to 
neutralise any lime they may contain. They also give to the 



20 PRINCIPLES OF LEATHER MANUFACTURE 

liquors a characteristic sour taste, as a consequence of which 
Hquors containing acetic and lactic acids are usually known in 
the tannery as " sour liquors." 

It is doubtful whether the action of fungi is completely stayed 
even by the dr5dng process. The heating of leather in the sheds 
is due to bacteria and the higher moulds, and Eitner considers 
their growth one of the causes of the " spueing " or " gumming " 
of curried leathers. 

From what has been said, it is obvious that, with regard to 
fermentations, a double problem is presented to the leather 
manufacturer, since he desires to utilise those which make for 
his advantage, while controlling or destroying those which are 
injurious. The first step to a solution of these problems is a 
more complete knowledge of the organisms which serve or injure 
us, that we may, as it were, discriminate friends and enemies. 
We may then approach the question in two ways. Taking the 
drenching process as an example, we may, on the one hand, intro- 
duce a " pure culture " of the right ferment into a sterilised 
bran infusion, and so induce only the one fermentation which we 
require ; or, on the other hand, as different ferments are affected 
in varying degrees by antiseptics, we may perhaps choose such 
as permit the growth of the organism we want, while killing or 
discouraging the rest. We may also arrange the nutriment, 
temperature, degree of acidity, and other conditions so as to 
favour one organism rather than another. All three methods 
have been applied in brewing with good results. 

Some iniormation on the technique of bacteriology may be 
found in chaps, xiv. and xv. of L.C.P.B. 



CHAPTER V 

ANTISEPTICS AND DISINFECTANTS 

" Antiseptics " are often defined as substances which check 
putrefaction without necessarily destroying bacteria and their 
spores, while " disinfectants " are poisonous to ferment- 
organisms, and actually destroy them ; but though great differ- 
ences exist in the extent of their sterilising power, the whole 
distinction is one rather of degree than of kind, and has little 
practical value. Thus common salt is incapable of killing most 
bacteria, even in concentrated solution, though it holds putre- 
faction in check both by withdrawing water from the hide and by 
directly preventing the multiplication of bacteria. If the salt 
be washed out of the hide, putrefaction is at once resumed by 
the organisms present. Hides, on the other hand, which have 
once been sterilised by powerful disinfectants, such as phenol 
(" carbolic acid ") or mercuric chloride, do not again putrefy till 
the' organisms which are killed are replaced by fresh ones from 
outside. The action of sodium sulphate, and many other salts, 
is similar to common salt in this respect, while a large proportion 
of the aromatic compounds are permanently disinfectant, though 
their efficiency varies with the species of bacteria involved. 

Biernacki and others have shown that some disinfectants when 
extremely diluted actually stimulate alcoholic fermentation, e.g. 
mercuric chloride i in 300,000, salicylic acid i in 6000, and boric 
acid I in 8000, and this is probably true of other ferments, and in 
many cases organisms become habituated to antiseptics in doses 
which would at first" have proved fatal. 

The number of antiseptics available is now so great that it is 
impossible to give a detailed account of all, but the following 
are among those which are best known and have been practically 
employed : — 

Lime possesses some antiseptic properties, and is largely used 
in the preservation of fleshings before they are sent off to the 
glue factory. They are most conveniently stored in a large vat 
filled with a strong and fresh milk of lime. Dilute solutions of 
caustic alkalies have an effect similar to that of lime. One per 
cent, solution of caustic soda is practically sterile for most bacteria, 
but when lime liquors have become charged with dissolved 



22 PRINCIPLES OF LEATHER MANUFACTURE 

organic matter, they support bacterial life ; and this is probably 
true of other alkaline solutions. 

Common salt, sodium chloride, NaCl, acts to a certain extent 
by its solubility and its dehydrating effect on animal tissues, 
which removes water from hides and other materials which it 
is used to preserve. Probably the latter characteristic has a 
good deal to do with its effect in checking the development of 
bacteria, since many species thrive quite well in weak salt 
solutions, and some even in brine, and the dehydrating effect of 
the salt enables it to harden many animal tissues if used in suffi- 
cient quantity, the water they contain running away in the form 
oi brine. 

The very considerable antiseptic and dehydrating effect of 
neutral salts like sodium chloride and sulphate on the hide seems 
at first sight somewhat inexplicable, and though in the case of 
the dry sulphate much water is removed as water of crystallisa- 
tion, this does not apply to common salt, and especially when it 
is used in the form of brine, since strong solutions have rather 
a swelling than a dehydrating effect on neutral gelatine, and, 
presumably, on hide-fibre. The explanation is, probably, that in 
the early stages after slaughter the skin is always acid from the 
production of sarcolactic and perhaps other acids, but as putre- 
faction proceeds it becomes alkaline from the evolution of 
ammonia. The result is that on the acid skin a slight " pickling " 
effect is produced (p. 234). No doubt a similar effect, but much 
less marked, may be produced by the Na' on the alkaline skin, 
but this explanation if correct points to the importance of early 
salting, and it may be suggested that if the skins are already 
slipping and alkaline, a more preservative effect, and probably 
an economy of salt, would be obtained by the addition of, say, 
2 to 3 per cent, of ground nitre-cake (hydric sodium sulphaite) 
to the salt to restore the acid condition. 

Ordinary rock salt frequently contains ferric oxide and some- 
times chloride ; and iron, either originally present in the salt, or 
derived from the action of the latter upon the iron contained in 
the blood, is one of the causes of what is known as " salt-stains." 
These are sometimes visible on the flesh of salted hides, but show 
little during the liming of the hides, unless sulphides are used, 
when stains appear of a greenish-black, from the formation of 
sulphide of iron ; and when the hides come into the tanning 
liquor, black or bluish stains are produced by the action of the 
tannin, which are partially removed by the acids of the liquors 
during the tanning process, but generally show to some extent 
in the finished hide. There are other species of salt-stains, not 



ANTISEPTICS AND DISINFECTANTS 23 

apparently due to iron, but to the colouring matter produced by 
fungoid or bacterial growth, which it is practically impossible to 
remove, and which are stated to be sometimes caused by the use 
of old salt with which hides have been previously salted. ^ Iron- 
stains are most readily recognised by the use of a solution of 
potassium ferrocyanide slightly acidified by hydrochloric acid. 
If this be applied to the leather, the stains will be changed from 
a blackish to a blue. A more absolutely conclusive proof is to 
lay a piece of filter paper soaked in dilute hydrochloric acid upon 
the stain, and then to test for iron upon the paper with ferro- 
cyanide or thiocyanate. The freedom of the paper itself from 
iron must be ascertained before use. Iron-stains produced in 
the salted state are more difficult to discharge than those which 
are caused later in the tanning process, since iron salts have 
distinct tanning power, and attach themselves firmly to the 
untanned fibre. On the Continent, where common salt is heavily 
taxed, alum, carbolic acid, naphthalene, and other materials are 
frequently added to it to " denaturise " or render it incapable of 
being used as food, and these additions are often the cause of 
trouble to the tanner. Sodium carbonate seems on the whole 
the best denaturising material. 

Sodium sulphate, Na2S04, has little if any disinfectant power 
in dilute solution, but if used in the calcined form (anhydrous 
sodium sulphate), as proposed by Eitner ^ as a substitute for 
common salt in preserving hides, it withdraws water from the 
hide and crystallises with 10 Aq (about 56 per cent.). This does 
not run away like brine, bat remains in the hide, which retains 
its weight, and remains plump and swells well in the limes and 
liquors, as the sulphate is partially converted into caustic soda 
by the action of the lime ; 10 to 15 per cent, on the weight of the 
hide is sufficient, while salt must be used in nearly double this 
quantity. Care must be taken that the sulphate used is free from 
bisulphate, NaHS04, which has a powerful swelling effect upon 
the hide-fibre, like sulphuric acid. The neutral sulphate does 
not redden methyl orange or litmus. 

The stronger mineral acids have considerable antiseptic power, 
even when very dilute, and are of course especially fatal to such 
ferments as thrive best in alkaline solutions. The use of sul- 
phuric acid in pickling skivers has already been aUuded to, and 
a very dilute solution applied without salt to raw hides prevents 
putrefaction, though the principal object in using it is to plump 
the hides and produce a fictitious weight and substance which 

^ For a further discussion of salt-stains see Chapter VI. p. 36. 
^ Gerber, 1880, p. 185. 



24 PRINCIPLES OF LEATHER MANUFACTURE 

disappear on tanning. Such hides of course have a powerful 
acid reaction to litmus. Sulphuric acid in small quantities has 
been used with advantage in soaking E.I. kips. A very small 
excess of hydrochloric acid will sterilise putrid effluents, and no 
doubt nitric or sulphuric acid would have the same effect. The 
powerful effect of mineral acids on animal fibre, and their solvent 
action on cements and iron, preclude, however, their general use 
as antiseptics. 

More important is the use of sulphurous acid and sulphur 



Ste^m m 



Water 
<iin.pipe 




Fig. 5. — Sulphurous acid apparatus. 



dioxide, which, from their mild acidity and great antiseptic 
powers, are capable of a variety of useful applications. Consider- 
able doubt has been raised as to the germicide power of sulphur 
dioxide, and it is certain that the dry gas is less effective on dry 
objects than when applied in solution or to moist materials, as 
is almost invariably the case in the tannery. It may possibly 
be more efficient in its action on some moulds and putrefaction- 
ferments than on the pathogenic bacteria which have been most 
frequently used to test the power of disinfectants ; but in practice 
it is found extremely useful in the brewery and in gelatine manu- 
facture, and there is no reason that it should be less so in the 
tannery. 

The gas is most conveniently produced by burning sulphur, 
which produces double its weight of sulphur dioxide. If used for 
" stoving " drying rooms and other places infested with moulds, 



ANTISEPTICS AND DISINFECTANTS 25 

care must be taken to avoid risk of fire. A shallow cast-iron 
pot set on bricks or sand is generally the most suitable vessel, 
and the sulphur may be ignited by a piece of red-hot iron or a 
rag which has been previously dipped in melted sulphur. It is 
corrosive to metal-work, and bleaches many colours, but does 
not produce any marked injurious effect on leather, though the 
sulphuric acid formed by oxidation may, if not removed, ulti- 
mately make it tender. 

For many purposes a solution of the gas is required, and this 
is most easily made by burning the sulphur in a small metal or 
firebrick stove from which the fumes are sucked through a 
" scrubber," which, on a small scale, is conveniently made of large 
glazed sanitary pipes, packed with coke ^ or broken earthenware, 
over which water is allowed to trickle. The lowest pipe has an 
opening for a branch pipe, which is connected with the stove, and 
rests on three bricks in a trough or tub, which collects the acid 
solution and forms a water-seal to prevent the escape of gas. 
Above the inlet for the gases is fixed a wooden grating, on which 
the coke rests. The scrubber may be 10 to 15 feet in height, and 
connected at the top with a chimney or steam ejector ^ to produce 
the draught. The arrangement is illustrated in fig. 5. Another 
method is to burn the sulphur in a closed cylinder and to force 
the products through water with an air-compressor, and this is 
necessary where the gas is used, as is now commonly done in 
reducing bichromate solution for chrome tanning (see p. 267). 

In place of using a scrubber, the fumes may be blown by a 
steam injector direct into a tank. This is a very good arrange- 
ment for washing and bleaching hair, etc., but where large 
quantities of solution are required is inferior to the scrubber. 
Injectors of hard lead or regulus metal should be used, and are 
less acted on by the dry gases than by the very dilute moist 
exhaust from the scrubber (see p. 401). 

Bisulphites have also strong antiseptic properties. " Bisul- 
phite of soda " (hydric sodic sulphite) solution may be made by 
supplying the scrubber with solution of soda-ash or washing soda ; 
bisulphite of lime, by using milk of lime or packing the scrubber 
with chalk or limestone (free from much iron) in place of the coke. 
In either case a much stronger solution is obtained than with 
water alone. 

^ Coke contains a good deal of iron, which will contaminate the acid 
for some little time. Broken pumice is free from this defect. 

2 The ejector must be of regulus metal to resist corrosion. The air 
admitted to the stove can be regulated by a brick placed in front of the 
opening. 



26 PRINCIPLES OF LEATHER MANUFACTURE 

Boakes' " metabisulphite of soda " Ws a very convenient 
source of sulphurous acid when the latter is wanted in small 
quantities. It is an anhydrosulphite, NagO. 2(802), ^nd contains 
67-4 per cent, of its weight of SOg. One molecule of the salt 
(=:igo) requires one molecule of HgSO^ (=98) to set free the 
whole of the sulphurous acid. For many purposes the sulphate 
of soda formed may be neglected and the acidified solution used 
direct. 

For analysis of sulphites and sulphurous acid solution see 
L.I.L.B., p. 73, and L.C.P.B., p. 13. 

If bisulphites are used in tanning liquors, their effect in bleach- 
ing and making soluble the " reds " (phlobaphenes) present in 
many tanning materials must not be overlooked. This action 
improves the colour of the leather, but at the same time 
diminishes its solidity and weight, though it probably assists in 
the more complete utilisation of the tannins. In most " bleach- 
ing extracts " bisulphites are present in large excess, and not 
only keep the " reds " of the extract itself in solution, but dis- 
solve those of other liquors with which they are mixed, and of the 
leather itself, thus rendering it more porous. 

Boric acid, borax, and other borates are not very powerful 
disinfectants. They have no injurious action upon the skin, but 
to be effective require to be employed in pretty strong solutions, 
say, I per cent., and their comparatively high cost unfits them for 
general use as antiseptics in the tannery, though boric (boracic) 
acid is very useful as a drenching and deliming agent (see p. 
205, and L.I.L.B., p. 37). 

Mercuric chloride, corrosive sublimate, HgCla, is an extremely 
powerful antiseptic, preventing the growth of some species of 
bacteria in solutions so dilute as i in 300,000 (Koch), i in 14,000 
is disinfectant (Miquel), but its power varies very much upon 
different organisms (Jorgensen states that i in 400 is required 
to kill Penicillium glaucum), and it is unsuited for most purposes 
in leather manufacture, both from its extremely poisonous char- 
acter, and because it is rendered inactive by various substances 
present in the materials used. In conjunction with formic acid, 
however (Seymour- Jones, p. 236), it is extremely effective in 
the destruction of anthrax spores, and can be employed in 
solutions so dilute as to be quite harmless to the workmen. 

Mercuric iodide dissolved in iodide of potassium solution 

was patented by Messrs Collin and Benoist as an antiseptic in 

tanning, but it is expensive, and has the same defects as mercuric 

chloride ; although under favourable circumstances it is even 

^ Patented by Boakes, Ltd., Stratford, London, E. 



ANTISEPTICS AND DISINFECTANTS 27 

more powerful than the latter. Mercuric and copper salts must 
not be dissolved in zinc or galvanised pails. 

Copper sulphate, zinc chloride and sulphate, and many other 
metallic salts are powerful antiseptics, but have only a limited 
application in leather industries, and do not usually actually 
sterilise. Copper sulphate is very fatal to algcB in water. 

Arsenic (arsenious acid), which has been used in curing 
hides, is an excellent insecticide, but is not effective as an 
antiseptic ; and sulphide of arsenic (realgar) when used in limes 
(see p. 189) seems to have little or no antiseptic effect. Arsenious 
acid is easily soluble in alkaline solutions. 

Fluorides have been suggested as antiseptics in the tannery, 
but have not been much used. 

The most important antiseptics at present are those 
derived from coal tar, and belonging to the aromatic series. 
Of these, the phenols (carbolic acid, cresol, etc.) are the most 
used. 

Pure phenol, " pure crystallised carbolic acid," is hydroxy- 
benzene, C6H5(OH), but the hquid forms which are generally 
employed contain cresols and higher members of the series, in 
which one or more of the atoms of hydrogen are substituted by 
CH3 groups. These are oily bodies scarcely soluble in water, 
and even pure phenol is only soluble in cold water to the extent 
of some 7 per cent. Crude carbolic acid should not be employed 
in the tannery, since the insoluble oily particles stain the hide, 
and render it unsusceptible of tanning. Suitable carbolic acid 
should be of a pale yellow colour when fresh (though it will 
darken on exposure to air and light), and it should be wholly 
soluble in a sufficient quantity of water. Its specific gravity 
should be 1-050 to 1*065. For methods of chemical examina- 
tion, see L.I.L.B., p. 78. A saturated solution of carbolic acid 
sterilises hide completely against most putrefactive organisms, 
but has a sort of tanning effect, adhering obstinately to the fibre 
so that it cannot be removed by washing ; and hides which have 
been cured with it cannot be unhaired by sweating, though they 
may be limed in the usual manner, if somewhat more slowly. 
Care should be taken in mixing with water or liquor, as un- 
dissolved drops will produce the same effects as those of the 
crude acid. Though heavier than water they frequently float, 
being sustained by surface-tension. Hides are occasionally 
stained in this way by salt which has been denaturised with 
common sorts of carbolic acid. Eitner recommends the use of 
a solution of carbolic acid in an equal weight of crude glycerine, 
which readily dissolves in water, and seems to prevent any 



28 PRINCIPLES OF LEATHER MANUFACTURE 

injurious effect on the hide ; but the present price of glycerine 
is prohibitive. 

An aqueous solution containing i per cent, of carbolic acid is 
sufficient for mere sterilising of hides, but if it be desired to 
preserve them for a long period, stronger solutions (up to 4 per 
cent.) ma3^ be employed. 

-Quantities so small as i part per 1000 control the fermenta- 
tion of liquors, and prevent the formation of moulds on the 
surface, economising tannin, and preserving vegetable acids 
already present, but at the same time lessening their production 
by feiTnentation, and therefore sometimes leading to difficulties 
in the early stages of tanning. Carbolic acid is not, strictly 
speaking, an acid, but rather of the nature of an alcohol, although 
it forms weak combinations with bases. It does not swell hides, 
but is a powerful narcotic poison, and if dropped on the skin 
in a concentrated form it produces severe burns, which are best 
treated with oil, while in cases of poisoning oil and chalk must 
be administered internally, but if the quantity of carbolic acid 
taken has been large, are not likely to be effective. From its 
cheapness and efficiency carbolic acid is likely to be increasingly 
used, although for special uses some of the newer antiseptics have 
great advantages. 

Creasotes and cresols can be dissolved in oils and stuffing greases, 
and act as antiseptics, though less powerfully than in aqueous 
solution. " Eudermin " is a preparation made for this purpose. 
Rosin oils and turpentine have also antiseptic properties. 

Creasote, " heavy coal oil," or " dead oil," is a complex mixture 
of hydrocarbons, phenols, and cresols, obtained by distillation of 
coal tar, heavier than water, and almost insoluble in it. It is 
largely used as a preservative for timber. " Carbolineum " is 
an oil of this class, boiling at over 300° C, and intended for 
application to wood. One or more coats are applied to the dry 
wood at a temperature of 80° C. The workman's hands must be 
protected by gloves, as the hot creasote raises painful blisters. 
Eitner ^ recommends its use for preserving pits, posts, and other 
woodwork in tanneries. Wood-creasote is a somewhat similar 
product obtained from wood-tar. 

The heavier cresols are so little soluble in water as to be value- 
less as antiseptics in their ordinary form, but several preparations 
are made under the names of " Creolin," " Jeyes' fluid," " Lysol," 
" Izal," " Soluble phenyl," etc., in which they are treated with 
additions of soap or alkalies, which cause them to emulsify or 
dissolve in water, generally as milky liquids, which are powerful 
1 Gerber, 1889, p. 183. 



ANTISEPTICS AND DISINFECTANTS 29 

germicides, and have the advantage over phenol of being non- 
poisonous, o-i to 0-5 per cent, solution of creolin will sterilise 
hides after bating so that no putrefaction takes place in the 
liquors. Mr J. T. Wood specially recommends creolin for the 
general purposes of the tannery, disinfecting pits and tubs, and 
for checking the action of puers and drenches on goods which 
have gone a little too far, by throwing them into a 0-2 per cent, 
solution. 

Salicylic acid, orthohydroxybenzoic acid, C6H40H(COOH), is 
now artificially prepared from phenol. It is much less poisonous 
than the latter and has no smell, which makes it valuable for 
certain purposes, but is too dear for most technical applications. 
Many bacteria appear to become gradually habituated to its 
action, and the same is true of phenol to a less degree. 

Salicylic acid is closely related to protocatechuic and gallic 
acids, and, like these, gives a blackish colour with iron salts. It 
is freely soluble in hot water, but very sparingly in cold. The 
addition of i to 2^ parts of sodium phosphate, sulphate, or potas- 
sium nitrate to each part of salicylic acid greatly increases its 
solubility. It is inore powerful in preventing the development 
of bacteria than carbolic acid ; a solution of i part of salicylic 
acid in 666 of water is said to be equal in this respect to i part 
of carbolic in 200. 

Benzoic acid, CgHgCOOH, though not much employed except 
in medicine, is a still more powerful disinfectant, and has the 
advantage of being non-poisonous to human beings. 

" Anticalcium " ■•• is a solution of mixed sulphonic acids derived 
from cresols, and has considerable disinfectant powers. It 
also removes hme very effectively, but from its acid character 
somewhat swells the skin. 

" C.T." (coal tar) bate is a grey crystalline pasty mass, with a 
tarry smell, and is chemically very similar to anticalcium, if not 
identical with it. Many other coal-tar sulphonic acids have 
similar properties. These preparations have not much im- 
portance for deliming, but may be useful where an antiseptic 
action is also required. 

Naphthols, CioH7(OH). — These bodies, which have the same 
relation to naphthalene as the. phenols to benzene, are powerful 
antiseptics ; and naphthalene itself appears to have antiseptic 
power, and is occasionally used for denaturising salt. There 
are two naphthols, varying in the position of the OH group in 
the molecule, and denominated a and j8, of which a naphthol is 
the more powerful antiseptic and the less poisonous, though j8, 
1 Gerber, 1895, p. 133. 



30 PRINCIPLES OF LEATHER MANUFACTURE 

being cheaper, is the common commercial article. It is said that 
quantities so small as o-i to 0-4 gram of a naphthol per liter are 
sufficient to prevent the development of microbes, -while of j8 
naphthol about ten times that quantity is required. 

Naphthols are not very expensive, but their value is diminished 
by the fact that they are insoluble in water, though soluble in 
alkaline solutions, but their compounds with bases are of much 
lower antiseptic value, and the same is true of their alcoholic 
solutions ; when an alcoholic solution is added to water the 
naphthol is precipitated, but if an addition of soap or camphor 
be made to the alcoholic solution, the naphthol remains in a very 
finely divided condition, if not dissolved. 

Adopting a suggestion of Eitner's ^ with regard to oxynaphthoic 
acid, hides may be sterilised by the naphthols and other difficultly 
soluble aromatic antiseptics by treatment first with a weak 
alkaline solution of the antiseptic, and afterwards with a weak 
acid to remove the alkali. Hides treated in this way are per- 
manently sterilised, and cannot be unhaired by sweating, and 
would probably lime with some difficulty. 

Carbon disulphide. — Moret has suggested an aqueous solution 
of this compound as an antiseptic, and it seems to have consider- 
able sterilising powers, but from its inflammability, poisonous 
character, and unpleasant smell, it is not likely to come largely 
into use. It is sometimes useful for preserving putrescible 
matter in the laboratory. 

Formaldehyde, COHg, in aqueous solution containing about 40 
per cent, of formaldehyde together with a little formic acid, is 
sold under the names of " formalin," " formol," etc. It has 
great disinfectant powers, and is valuable in various processes 
of leather manufacture, but has a hardening and tanning effect 
on hide-fibre and gelatinous matters, so that in very dilute 
solution it will produce leather.^ The vapour of formaldehyde, 
or of its condensation-product paraform, may be employed to 
harden microscopic preparations. One part of formaldehyde, 
and consequently 2| parts of " formalin " in 12,000 parts of 
water, is said to sterilise, and forms a good disinfectant solution. 
Even in considerably larger proportion than the above it is not 
poisonous, and thus possesses the bactericidal power of sublimate 
without the latter 's poisonous properties. Formaldehyde has 
another advantage over most, if not all, other antiseptics, in that 
it may be used as well in the gaseous as in the liquid state, and on 
that account it is largely employed in the disinfection of rooms 

^ Gerber, 1888, p. loi ; 1889, pp. 99 et seq. See also p. 206. 
^ Gerber, 1897, p. 67 ; ibid., 1899, pp. loi, 205, 218. 



ANTISEPTICS AND DISINFECTANTS 31 

or of articles which would be spoiled if they were to be wetted, 
as the gaseous formaldehyde, though thoroughly disinfecting 
them, will not injure the colours of materials of the most delicate 
fabrics. 

On account of its power of rendering gelatinous matters hard 
and insoluble in water, formaldehyde requires to be employed 
with great caution in leather manufacture, but o"2.to 0-3 per cent, 
may be successfully used in admixture with egg-albumen in the 
preparation of " seasoning " in the finishing of morocco leather. 
It is also used commercially to produce different varieties of white 
leather for soldiers' accoutrements and similar purposes (pp. 459, 
576), and for fixing hides in a swollen condition prior to tanning. 

Triformol (tri-oxym ethylene, " paraform ") is a product of 
the polymerisation of formaldehyde, and is prepared by evapo- 
rating a solution of the latter to drjmess on the water-bath. It is 
more powerful than formalin in its antiseptic properties, but has 
not entered very largely into use as a disinfectant, though con- 
siderable use is made of it to " fix " bacteria in gelatine for 
bacteriological purposes. 

Camphor and essential oils, as well as oil of turpentine, are 
antiseptics, and the cheaper essential oils, such as those of winter- 
green, black birch, sassafras, and aniseed, are frequently em- 
ployed, especially in America, in preserving pastes, finishes, and 
seasonings, and at the same time covering offensive odours. The 
odour of essential oils becomes much more powerful as they are 
diluted, and very small quantities suffice for the purposes men- 
tioned. Birch-tar oil, such as is used to give the scent to Russian 
leather (p. 453), is antiseptic, and prevents the attack of insects. 
Artificial methyl salicylate is now made, which is chemically 
identical with oil of winter-green, though from the presence of 
some impurity it has a different, but not unpleasant, odour. It 
has considerable antiseptic powers, and is much used as an external 
rheumatism remedy. The essential oils of cloves and pimento 
are also good antiseptics. 



CHAPTER VI 

THE ORIGIN AND CURING OF HIDES AND SKINS 

A CONSIDERABLE proportion of the hides and skins used in 
leather manufacture are those of animals killed by the butcher 
for food, and these are frequently employed by the tanner with- 
out any preliminary curing. Domestic hides and skins are 
now generally sold by auction in weekly markets in the principal 
towns, after sorting and classification in weight and quality. ^ 
This is in many respects an improvement on the old method of 
purchase direct from the butcher, but it often leads to delaj^ in 
delivery, and in hot weather hides suffer from putrefaction. In 




Fig. 6. — Method of marking weight on hides ; 97 lb. 

most cases the damage is not suf&cient seriously to affect the 
durability of the leather, but the delicate membrane of the 
" grain " is injured, and the hide or skin unfitted for coloured 
leather, or any purpose where small damages to appearance are 
important. Butchers are averse to the use of salt, because it 
withdraws water from the hide in the form of brine, and so 
causes it to lose weight ; but much injury would be saved by 
a light salting, and all hides or skins on which the hair is " slip- 
ping" should be regarded as damaged for fine leather manufacture. 
Sheep-skins are not usually bought direct by the tanner, but 
by the fellmonger, who removes the wool ; and as this is of much 

^ The weight of Enghsh market-hides as credited to the butcher is 
usually marked on the edge of the butt near the tail by cuts with a knife, 
the mode of numeration being sufficiently explained by Fig. 6, in which 
cuts crossing the horizontal line each represent 20 lb., that above it 10 lb., 
while less amounts are expressed in Roman figures. 

On the Continent weights are usually given in pounds of half a kilo- 
gram (50 kilos = iio lb. English). In Paris the marking is on the tail, 
and is also shown on fig. 6. 

32 



ORIGIN AND CURING OF HIDES AND SKINS 33 

greater value than the skin, the latter is frequently handled very 
carelessly, and its quality sacrificed for the sake of real or fancied 
improvement to the wool. In very many cases the skin is 
" sweated " or " staled " by hanging in a warm and moist 
chamber, heavily charged with ammonia derived from the putre- 
faction of the skin, until the wool is sufficiently loosened to be 
" pulled." If this treatment is conducted with extreme care 
the skin may escape serious injury, but in most cases the grain 
is weakened, and the foundation is laid of damage, which makes 
itself felt throughout the tanning process.^ 

The best method is that generally employed by the freezing 
companies, and to some extent by the more modern fellmongers, 
in which the skins, after thorough washing, are painted on the 
flesh side with a thick milk of lime, to which is added 25 to 30 
per cent, of sulphide of sodium on the weight of the lime employed, 
taking care that none of the solution touches the wool. The 
skins are then piled flesh to flesh for twenty-four hours, when 
the wool will be found ready to puU. An older method is to use 
milk of lime only, and in the case of shearlings, where the wool 
is not of so much value, the skins are laid flesh to flesh in shallow 
pits and covered with water. For further details see Chapter 
XIII. on depilation. 

One of the great causes of injury to the pelts is their collection 
by the fellmonger in what are called " gathering limes," where 
they remain till taken by the pelt sorter. These limes are 
frequently very old and stale, and swarming with putrefaction 
bacteria. If kept in limes at all, these should be fresh and fre- 
quently renewed, but even fresh sweet limes dissolve the cement- 
ing substance of the fibres, and increase the naturally loose 
texture of the sheep-skin if the treatment is long continued. In 
the American stockyards the skins are only limed for the time 
necessary to swell and differentiate the fibre, and are then at 
once drenched and preserved by pickling (p. 234). 

Where hides or skins cannot be used at once in the fresh state, 
there is probably no better method of preserving them than the 
use of salt. Although salt is not fatal to bacteria, it so slows 
bacterial growth, partly by its direct antiseptic effect on many 
organisms, and partly by withdrawing water from the skin, 
that weU-salted skins can be kept in good condition for an almost 
unlimited time. Where it is only required to preserve goods for 
a week or two, a moderate sprinkling on the flesh side is efficient, 
but if they are to be preserved for a length of time, more thorough 

^ As to the use of ammonia gas as a substitute for putrefaction see 
p. 166. 

3 



34 PRINCIPLES OF LEATHER MANUFACTURE 

treatment is necessary. It is said that however carefully hides 
are salted, they deteriorate if kept in this condition above twelve 
months. It is advantageous to wash and well-drain hides before 
salting, as by this means most of the blood, lymph, and dung 
are removed, which are the most putrescible constituents of the 
hide. 

The method of salting employed in the Chicago stockyards 
for " packer " hides may be taken as a good type of a thorough 
salting. The hides are first trimmed from useless " switches," 
and any large portions of adhering fat are removed. The curing 
takes place in large and cool" cellars with concrete floors. The 
detail is well given in the following extract from the Shoe and 
Leather Reporter : — 

" Great care is taken to make the sides of a pack higher than 
the middle, so that the brine which is made by the juices of 
the hide coming in contact with the salt will be retained. The 
brine can only escape by percolation, and hence the fibre of the 
hides is thoroughly cured. The floor of a hide cellar is usually 
of concrete, and a pack is from 15 to 20 feet long, and as wide as 
the space between the posts which support the floor above. The 
sides of a pack are built first to a height of from 4 to 6 inches ; 
the cross layers are then put on, generally three on each side, 
two being inside and one having the butts drawn out to the 
edge. In a pack 20 feet long the side layers will contain about 
25 medium-sized hides each, and a cross-layer 12 or 14. To 
begin a pack a truck-load of hides is run along to the front of 
the place selected, one spreader grasps the butt and his partner 
the head of a hide, and together they carry it to what is to be 
the rear of the bed. The hide is then dropped, so that the folded 
back is parallel to and from 15 to 20 inches from the inside line 
of the posts, the head a trifle closer than the butt. The front 
man takes the dewlap and front shank in his left hand, and 
extends his right along the belly of the hide as far as is necessary 
to raise the edge, the rear man holding the flank with one hand 
and the hind-shank with the other. They keep their legs well 
out of the way of the salt thrower, who with a single throw 
covers the whole hide, being particular that enough salt strikes 
against the edges held, by the men to make a pronounced ridge 
when they are lapped down. A little salt is thrown on the hair 
surface and the butt folded over about a foot. The folded edge 
is then drawn out even with the outer line of the pack. More 
hides are placed the same way until the corner is high enough. 
After this, each hide is put further forward to make a level 
surface from rear to front, the heads at the front corner being 



ORIGIN AND CURING OF HIDES AND SKINS 35 

folded back as the butts were at the starting place. The other 
side is built the same way, and then the cross layers are put on 
alternately until the pack is level, when sides are again built as 
before. In putting on the first hides of the cross layers they 
are thrown over the edge, to lap back again when the salt is 
thrown on ; the layer is then continued on to the front. The 
spreader who holds the butt does the guiding in every case. 
He drops the butt down at exactly the proper place, takes the 
upper flank and shank in each hand, sets one foot on the lower 
shank to keep it firm, and throws the one in his hands from him 
with considerable force. The man at the head watches his 
partner, keeps the folded hide taut, and drops it at the same 
time as the latter. He takes the fore-shank at the knee in the 
one hand and the upper headpiece in the other, and setting his 
foot on the lower side, throws the upper side forward simultane- 
ously with the rear man. Two expert spreaders, accustomed to 
working together, spread a hide at a single throw, but some little 
straightening has to be done by hand before the hide is ready 
for the salt. A gang composed of two spreaders, one salt thrower, 
and a salt trucker put down forty hides an hour. When gangs 
are doubled, two men do all the spreading ; the other two place 
the hides where they can be got at conveniently. A double 
gang put down eighty hides an hour. The salt trucker brings 
the salt to the pack in box-trucks, open at one end to permit the 
entrance of a shovel. The salt thrower keeps the edges and 
corners of the pack full of salt. He must see that every part of 
the flesh side is well covered. Each hide takes two scoop 
shovelfuls of ground rock or coarse white salt, mixed with an 
equal quantity of old or second salt. The salt thrower throws 
the shovel forward and to one side and back again with a peculiar 
swinging jerk, causing the salt to fall regularly over the entire 
surface of the hide. The ease and rapidity with which a gang 
operates depends greatly upon the efficiency of the salt thrower. 
When the pack gets too high to be comfortable for the men, it 
is brought to a dead level and covered over with clean salt. It 
then presents a very neat and workmanlike appearance. When 
the temperature is kept at an even average, two weeks is ample 
time to cure the hides. 

" In ' taking up,' two men strip the hides from the pack. As 
they were put down from the rear to the front, they are taken 
up in the reverse direction. No matter how much loose salt is 
lying on the top, the man knows exactly where to place his 
hand on a shank ; as the hides are moved forward, the loose 
salt is thrown off toward the front. One man takes away the 



36 PRINCIPLES OF LEATHER MANUFACTURE 

salt as it accumulates and trucks it to the salt bins, where it 
is mixed with new, to be used again. A ' horse,' made of a 
network of scantling about 3| feet wide by 6 feet long, and stand- 
ing 2| feet from the floor, is placed in front of the pack ; on this 
the hides, flesh side down, are shaken to remove the salt that is 
clinging to them. This process requires four men, one at each 
corner. The hide is brought down heavily on the horse twice, 
and then spread on the floor flesh side up for examination by 
the inspectors, of which there are two, one representing the 
house and the other the buyer of the hides. They sweep off 
any salt that may be left, and examine for cuts, sores, brands, 
manure, and grubs. They also see that the hide is properly 
weighed and classified. If the contract calls for a special trim 
it is now done. Two men then roll the hide, beginning by 
lapping over the shanks, head, and neck. Then the sides are 
folded over and lapped again, leaving the roll 15 to 18 inches 
wide. The ends are thrown inward, slightly overlapping each 
other ; a final fold is then given, and the hide is ready to be 
tied. Rope the size of clothes-line is used for tying, and is cut 
into lengths of about seven feet. It takes three men to tie for 
a gang such as we have described. After tying, the neat bundles 
are weighed and loaded on the cars for shipment. A small tare 
is allowed the buyer." 

About 25 per cent, of salt on the green weight of the hide is 
required for thorough curing. Rock salt, merely crushed, is 
frequently used, but this is very liable to contain iron in the form 
of ferric oxide, which is said to be the cause of the peculiar marbled 
or map-like markings known as salt-stains. These certainly 
contain iron, as is shown by their brownish-yellow colour when 
the hides are unhaired from a white lime, their dull green 
from sulphide limes, and the fact that they turn black or 
purplish in tan liquors, and give the ordinary blue with ferro- 
and ferricyanides, and it is certainly better to use crystallised 
salt, but its use does not always prevent them, and their 
cause is probably more complex, and not unconnected with 
bacterial action. In 1912 it was the subject of considerable 
investigation by Dr Abt, Professor Becker, and others, the results 
of which wiU be found in the volume of Collegium for that year, 
but unfortunately these authorities do not altogether agree in 
their conclusions, and it is possible that they were not dealing 
with the same defect, as probably various stains of different 
origin are confused under the general name. Abt found that 
the stains which he examined appeared to originate on the 
flesh side and spread through to the grain. He attributed them 



ORIGIN AND CURING OF HIDES AND SKINS 37 

to the small spherocrystals of double sulphate of calcium and 
sodium (Schlott's grains) which occasionally occur in salt, and 
which he sometimes found adhering to the spot where the stains 
originated. The stained parts did not usually contain more iron 
than the rest of the unstained skin, but, singularly, had a large 
excess of phosphoric acid, which is not contained in the salt. He 
produced similar stains artificially by placing on the flesh side 
grains of calcium sulphate with crystals of ammonium phosphate, 
and their appearance was favoured by the addition of traces of 
iron salts. He attributes the phosphoric acid (probably in the 
form of ammonium phosphate) to bacterial action on the nuclei 
of the epidermis and hair-sheaths, and the iron may be derived 
in sufficient quantity from the haemoglobin of the blood left in 
the skin. He admits that the colour-reactions with sulphides, 
tannin, etc., are due to iron, but cannot explain why the unstained 
parts, which contain sensibly as much iron, do not stain. He 
found no microscopic evidence of bacteria in the stains, and 
attempts at culture gave negative results. 

Becker, on the other hand, found in the ochre-yellow stains 
cocci of about i"5 /-t diameter, which he cultivated easily on meat- 
broth peptone gelatine, and which were aerobic (required oxygen) , 
and liquefied the gelatine, and gradually liquefy hide. He also 
detected coccus bacteria in the red and orange stains, but these 
did not liquefy gelatine or attack hide, and may be considered 
harmless to the tanner. They were also cultivated on bouillon- 
peptone gelatine, but with considerable difficulty, requiring a 
somewhat high temperature and a long time (about six weeks) 
for their development. With all three bacteria Becker found 
a torula or yeast, with which they appear to be symbiotic. The 
red and orange bacteria could be cultivated alone on raw hide 
containing blood and lymph, but not on pelt, from which these 
liquid nourishments had been removed by the wet-work, but 
developed on the latter when associated with the torula, of which 
the dead cells furnished them with nourishment. Becker con- 
sidered that salt alone is not sufficiently antiseptic to prevent 
the growth of bacteria, but found that an addition of not less than 
4 per cent, on the weight of the salt of calcined sodium carbonate 
rendered it practically sterile. He also found that soaking the 
hides in a 0*25 per cent, solution of zinc chloride for two hours, 
or in a 0'5 per cent, solution for half an hour, previous to salting 
had a similar effect. He stated that 1*5 per cent, addition of 
sodium perborate to the salt would sterilise but swelled the 
hides, and recommends soaking for an hour previous to salting 
in a 0*25 per cent, solution of artificial mustard-oil, but points 



38 PRINCIPLES OF LEATHER MANUFACTURE 

out that the hands of the workmen must be protected by india- 
rubber gloves. 

The writer has from time to time examined many salt-stains 
of the map-like kind which appear on the grain, but has never 
paid much attention to the flesh, nor seen signs of serious injury 
there. These marbled or map-like stains, whatever their original 
cause, always owed their colour to iron, and gave the usual iron- 
reactions. In this connection attention may be drawn to the 
case of plaster-kips mentioned on a later page. Although much 
iron was contained in the salt-earth which constituted the cure, 
serious staining only took place after the kips had been kept 
through a winter in a damp London warehouse, and the iron- 
tannage of the grain was then so bad as to render the unhairing 
very difficult and imperfect. In this case there was little doubt 
that the iron was derived from the cure ; but the writer has also 
observed marked iron-stains following the course of a vein con- 
taining blood, which is quite common in kips which have died 
from natural causes, and which are known as " dead " in dis- 
tinction from " slaughtered." 

It is impossible to anyone who has known them to doubt the 
care, accuracy, and competence of both Abt and Becker as 
chemists and bacteriologists, and it is much to be desired that 
some explanation should be found of their differences, though it 
may be feared that the effects of the war have rendered co- 
operative work for the present impossible. It may be that they 
were working on distinct kinds of stain, but it is not impossible 
that iron may have accumulated in the spots in consequence of 
the presence of phosphoric acid, with which iron forms a very 
insoluble compound, or, on the other hand, that Becker's 
bacteria owed their colour to iron. Bacteria are well known which 
absorb iron. The writer has a well on his premises which contains 
a trace of iron bicarbonate together with calcium and a little 
sodium bicarbonate, and its walls are always covered with a red- 
brown deposit of a special iron-bacterium. Though Abt does 
not specifically say so, he apparently made his analyses of the 
stained and unstained portions from the flesh side of the hide, 
and it is not improbable that the iron was simply attached to 
the unstained portions in an insoluble form (oxide or carbonate) , 
while in the spots it had become soluble and diffused into the 
hide. These, however, are speculations, and must wait ,further 
investigation. 

A reddening of the flesh side is often noticed in hides which 
have been kept in salt long or under unsatisfactory conditions, 
and is very frequent in wet-salted South American hides. Such 



ORIGIN AND CURING OF HIDES AND SKINS 39 

hides are said never to produce so firm a leather as those which 
are sound. This may be due to one of Becker's pigment 
bacteria. It is also conceivable that it may be caused by B. 
prodigiosus, the bacterium which causes red bread, etc., and 
which is known to attack pickled skins. 

Hides are frequently cured by steeping in salt brine instead 
of strewing with dry salt. This method is principally resorted 
to in order to give fictitious weight. Brined hides do not plump 
well in tanning, the leather is not so good in quality as from those 
salted with dry salt, and the cure is much less efficient. 

Many hides are not only salted but also dried in order to 
preserve them. Not much detail has been published with regard 
to the methods used, which no doubt vary much in different 
places, but probably in some cases the hides are salted in pile 
and in others by brining, and then hung up to dry. The principal 
object of drying is to economise weight and cost of transport, 
but it makes the hides much more difficult to wash and soften 
for tanning (though easier than hides dried without salting), and 
probably the crystallisation of the salt has a weakening effect 
on the fibre. Hides cured in this way are styled " dry-salted." 

A large number of the hides of the small native cattle of 
India are imported into this country in a dry-salted condition. 
The following particulars of their cure are taken from a paper by 
the Author and Mr W. Towse.-^ 

Dry-salted, or, as they are commonly called, " plaster cures," 
such as those of Dacca and Mehapore, are thickly coated with a 
white material, which in the first instance is merely the insoluble 
portion of a saline earth used in the cure ; though in many cases 
it is applied in larger quantities than necessary, with the simple 
object of giving weight. The salting is thus described by Mr 
W. G. Evans, who some years ago had considerable experience 
as a tanner at Cawnpore :■ — • 

" The salt used by the natives is a salt-earth, and is so called 
by them. It is found extensively in the districts of Cawnpore, 
Agra, Delhi, Lucknow, Patna, etc., and has no doubt something 
to do with the localisation of the hide-curing and kindred in- 
dustries in these places. The mode of procedure used is pretty 
much as follows : the salt-earth is mixed into a very thin paste, 
and this is lightly brushed on to the flesh side one day, and the 
hide allowed to remain over night under cover. Next day, for 
best hides, the same solution is again spread on the flesh side of 
the outstretched hide and rubbed into it with a porous brick, and 
then, for legitimate salting, the hide is allowed to dry under 
^ Journ. Soc. Chem. Ind., 1895, p. 1025. 



40 PRINCIPLES OF LEATHER MANUFACTURE 



cover. If for export, the saltings may be three or four, and the 
hides are treated out in the open, subject to the intense heat of 
the sun ; which accounts for the number of hides which go back 
in the soaks in England and elsewhere. We had a clause in our 
agreement with hide-factors, that any hides which did not come 
down to natural suppleness in two days in clean water were to 
be returned. Of arsenic curing I know nothing, and it is not so 
much in vogue as formerly. There is quite a trade in Cawnpore, 
Lucknow, Allahabad, etc., in treating old and inferior hides with 
new for export, and great efforts are made by native holders to 
get their stocks down before the rains commence, as they say, 
and rightly I think, that hides are not worth so much after the 
rains by 30 per cent. The peculiar latent moisture of the rains 
affects them very detrimentally." 

Under certain circumstances this mode of cure gives rise to 
extensive iron-staining of the skins, and analyses of the material 
scraped off Dacca and Mehapore kips were undertaken with a 
view to elucidating the causes of this injury. The following are 
the results of the analyses referred to, which were made upon 
the residue after the rather considerable quantity of fibrous 
organic matter, which had been scraped off with the cure, had 
been destroyed by ignition, together no doubt with traces of 
ammoniacal salts : — 



Sand and silica 

FegOg 

AI2O3 

Mn304 

CaO 

MgO 

NagO 

SO3 

CI . 

H.PO. 



and CO., 



Dacca. 


Mehapore. 


Entire Cure. 


Entire Cure. 


20-55 


27-38 


277 


1-86 


2-48 


2-74 


o-6o 


0-40 


2-6o 


370 


3-38 


3-69 


28-97 


26-80 


38-90 


3375 


0-22 


o-i8 


Traces 


Traces 



100-47 



100-50 



The soluble salts of the Dacca cure were also analysed separately 
with the following result : — 
CaO 



V^O-V/ .... 

MgO .... 


o-6o 


NagO .... 


29-00 


SO3 . . . . 


• 37-90 


CI ... . 


0-22 


Moisture 


. . . 32-12 



100-54 



ORIGIN AND CURING OF HIDES AND SKINS 41 

It thus consisted exclusively of sulphates, with a mere trace of 
chloride. The cures, after ignition, were both neutral to 
phenolphthalein, but before ignition the Dacca was distinctly 
alkaline, in consequence probably of the presence of ammonium 
salts, and both showed considerably larger traces of carbonates 
before than after. 

The most striking feature of these analyses is the absence of 
chlorides. The cures are thus practically free from common 
salt, and owe their antiseptic power to the sodium sulphate 
which they contain, and which indeed forms their principal 
constituent. Nitrates appear to be entirely absent. Sodium 
sulphate sometimes forms large crystals in pits used for soaking 
these kips. 

The iron-staining of hides which has been mentioned appears 
to result only when the hides after cure are exposed for a 
lengthened period to a moist atmosphere, in which the carbonic 
acid present probably also plays its part, the iron passing into 
solution as hydric carbonate. 

The analyses show a striking resemblance to those of the 
soda deposits of Wyoming, given by Dr Attfield,^ except that 
their percentage of sodium carbonate is smaller, which is quite 
intelligible in the light of Mr Brunner's abstract on the " Probable 
Origin of Natural Deposits of Sodium Carbonate," ^ which supports 
the view that the sodium carbonate is derived from sodium sul- 
phate by the reducing and carbonating action of low organisms. 

It may be noted here that the preservative properties of 
sodium sulphate are well known, and the anhydrous sulphate has 
been recommended as a substitute for common salt (see p. 23). 

Drying is a very common method of preserving hides as well 
as other putrescible matters. It has no effect in killing bacteria, 
but putrefaction can only go on in presence of a considerable 
amount of moisture. As applied to hides, it is, to the tanner, 
one of the least satisfactory modes of cure, involving very con- 
siderable difficulties in bringing hides back to the moist and 
swollen condition which is necessary at the outset of his opera- 
tions, but it is the only practical method in districts far from 
the coast and with primitive modes of transit, both on account of 
the cost of salt and the lessened weight of the dried hide. Great 
differences are found in the ease with which dried hides soften, 
according to the way in which the drying has been accomplished, 
the difficulty being greater the higher the temperature which has 
been used (see p. 159). The best mode of drying is to hang in 
the shade in a good draught of cool air, with the flesh side out. 
1 Journ. Soc. Chem. Ind., 1895, p. 4. - Ibid., 1893, p. 116. 



42 PRINCIPLES OF LEATHER MANUFACTURE 

Hides or skins dried in a tropical sun are not only difficult to 
soften, but are liable to damaged portions, which either refuse to 
soften or blister and go to pieces in liming, owing to the structure 
of the hide being destroyed by heat, the outer surface drying first 
and forming an impervious layer which hinders evaporation from 
the inside, so that the moist interior becomes melted, while the 
outside appears quite sound. Such injuries are often only to be 
discovered by soaking and liming. Very similar damage may 
occur from putrefaction of the interior after the outside has 
become dry, and to get good results the drying must be gradual, 
but rapid, especially in hot climates. South American hides 
are mostly dried in the sun, suspended by head and tail from 
stakes, with the hair side out. 

The risk of injury by putrefaction during drying is diminished 
by the use of antiseptics. Solutions of arsenic have been fre- 
quently used for this purpose, and many of the dried Indian kips 
are of what are known as " arsenic cures," although the writer 
has never been able to detect arsenic in any which he has 
examined, and its use seems by no means general. The arsenious 
acid is usually dissolved in soda solutions. Such solutions have 
little antiseptic power even if strong, and are mainly useful in 
preventing the attacks of insects, which are often very destruc- 
tive. The larva of a small beetle, Dermestes vulpinus, frequently 
devours the whole fibrous tissue of patches of the hide, leaving 
only the epidermis. 

It may be well here to say a few words about the injuries and 
defects to which hides and skins are liable, although some of them 
are not strictly due to the cure. The most serious, and yet pre- 
ventable, injury is that due to butchers' cuts. As the value of the 
hide bears only a small proportion to that of the meat, many 
butchers do their work extremely carelessly, and this is en-, 
couraged by the loose classification of " damaged hides " in some 
markets. There is also an idea that the appearance of the meat 
is improved by a thin layer of the white skin-tissue being left on 
it, and, for this reason as well as mere carelessness, butchers fre- 
quently score the flanks of the hide with shallow cuts, which 
greatly diminish its value. The " packer hides " of the United 
States, and the products of the large saladeros or slaughtering 
(" salting ") establishments of South America, such as Liebig's, 
show what can be done by skilled work in this respect. In the 
United States, much of the flaying is done by means of a wooden 
cleaver instead of a sharp knife. Another method to some 
extent in use, and which may be recommended for calf- and sheep- 
skins, is to inflate the carcase before skinning with air from a 



ORIGIN AND CURING OF HIDES AND SKINS 43 

compressing syringe, which tears the connecting tissue between 
the skin and the body, and renders flaying much easier. 

Brands are a great source of damage to hides, but where 
cattle roam at large on unfenced plains, as on the prairies of 
Texas and the Pampas of South America, it seems indispensable 
for the recognition of ownership, no other mode of marking being 
sufficiently permanent and conspicuous. It is unfortunate that, 
as the animals crowd together, and cannot be closely approached, 
it is necessary that the brands should not only be large, but 
placed on the most valuable part of the hide. Generally on the 
Pampas an effort is made to keep them on one side only, so that 
in South American hides it is possible to select clear and branded 
sides. In the United States much land is now fenced with 
barbed wire, which, while it obviates the necessity of branding, 
introduces another evil in the form of " barbed wire scratches," 
which are frequently troublesome in " packer hides." 

Tar-brands may also cause considerable injury to the grain, 
especially if applied with a very hot iron, though it is much more 
superficial than that of those actually burnt into the skin. 

In countries where cattle are used for draught purposes goad- 
marks are a frequent source of injury, and some of the large 
cattle-ticks do considerable damage to the hides of Spain and 
South America. From the tanners' point of view, however, the 
most injurious insects are the " bot-flies " or " warble-flies " 
{Hypoderma bovis and alUed species, fig. 8). There has been 
much controversy as to how the eggs of these insects are deposited. 
In the horse bot-fly it is known that the eggs, first deposited on 
the skin, are licked off and swallowed by the animal, and develop 
in the stomach, where they pass their larval and pupal life hang- 
ing on to its interior coats, and only drop off and are passed 
out with the dung before their final change to the complete fly. 
Fortified by this, and by some direct observation, some American 
naturalists have been for some years of opinion that the American 
species at least hatches in the stomach, and as a minute larva 
wanders through aU the intervening tissues till it reaches the 
skin, where it undergoes its further development. The late Miss 
Ormerod, who made a careful study of Hypoderma bovis} believed 

^ E. A. Ormerod, Some Observations on the (Estrid^, Simpkin & Marshall, 
London, 1884. 

Prof. Clement Vaney, Reports of the Association Franfaise pour la 
Destruction du Varron, Paris, 1910-1911 : Le Developpement de I'Hypo- 
devme du Boeuf, and Les Varrons d'hiver, Goury, Paris, 191 1. 

Dr Hans Glaser, Mittheilungen des Ausschusses zur Bekdmpfung der 
Dasselplage, Nr. 3-6, Berlin, 1912-14. 
; Department of Agriculture and Technical Instruction for Ireland : 



44 PRINCIPLES OF LEATHER MANUFACTURE 




Fig. 7. — Sac of warble, 
showing growth of epi- 
dermis round aperture. 



that the egg was laid on the hair of the back, where it hatched, 
and the minute larva simply ate its way below the skin. In 
1914 the true life-history is believed to have been discovered 
simultaneously by Carpenter in Ireland, Seymour-Hadwen in 
Canada, and Glaser in Berlin, and is even more extraordinary. 
The flies never strike on the back, where the maggots are found, 
but on the fetlock or joint below the knee, anatomically the heel 
of the animal. The H. bovis, the more commion English species, 

lays single eggs, while the H. lineata, 
more frequent in America, lays them in 
single rows, but otherwise almost like 
the grains in an ear of corn. The 
minute maggot, at] first only about ^-^ 
inch long, bores its way through the 
skin, and wanders through' the tissues 
till it reaches the skin of the back, 
where it passes its final larval stage, 
often pausing on the way for a time in 
the wall of the gullet, where it is frequently found from September 
to January.^ It then bores a hole through the skin, enlarged to 
obtain air for its spiracles or breathing holes, which are in the 
tail. As it grows it continues to irritate the lower part of the 
cavity with hooked mandibles, and lives on the pus and matter 
so produced. It grows to a length of fully f inch, and the cavity 
(fig. 7), situated between the skin and the subcutaneous tissue, 
is often as large as half a walnut. It remains in the sac only 
during its larval stage, and falls out on the ground before com- 
pleting the change to the pupal state, and seeks shelter in the soil 
or under grass, emerging in about six weeks as a fly. Glaser, 
with characteristic German thoroughness, allowed one to bore 
through his trousers into his leg, and finally recovered it from 
his mouth ! The H. bovis is shown in fig. 8, and the H. lineata, 
which is somewhat smaller, and the eggs, in fig. 8a. The tail- 
Prof. G. H. Carpenter and others. Reports on " Warble Flies," No. 4, 
July 1910 ; /otirwaZ, No. 2, Jan. 1908 ; No. 4, 1910 ; No. i, 1915. 'Dublin. 
Carpenter and Hewitt, Some New Observations on the Life-history of 
Warble Flies, Thorn & Co., Ltd., Dublin, 1914. 

Department of Agriculture, Canada, Health of Animals Branch : Dr 
Seymour-Hadwen and others. Bulletin No. 16, 1912, and No. 22, 1916. 
Government Printing Office, Ottawa. 

Canadian Entomological Society, Report No. 36, 1916. 
See also an excellent and well-illustrated article by A. Seymour-Jones, 
Leather Trades Year-Book, 1921, p. 169. 

^ This is less difficult than might be imagined, as the path is mostly 
through the loose areolar tissues which surround most of the internal organs. 



ORIGIN AND CURING OF HIDES AND SKINS 45 

end of the former is covered with orange-yellow hairs, while in 
the latter they are lemon-yellow. 

As regards preventive measures, the most certain is to destroy 
the grub before it emerges, either by squeezing it out, or extracting 
it with forceps, and this should be done at intervals from May to 
August. The method is not very rapid, and it took about six 
years to completely exterminate the fly on a small island taken 
for experiment on the Irish coast. No dip or application yet 




Fig. 8. — Hypoderma bovis. i, egg, magnified about 12 diameters; 2, 
maggot; 4, chrysalis case ; 6, fly, magnified (Brauer) ; 3, 5, chrysalis and 
fly, natural size (B. Clark). 



found seems to prevent the fly from striking, and though some 
of the washes which have been tried do kill the grub, they mostly 
injure the skin and destroy the hair. The most effective and 
least injurious seems to be a solution of nicotine, with which 
the Board of Agriculture and Fisheries are making further 
experiments. 

In small numbers the warble seems to do little injury to the 
health of the animal, though it must damage its condition to 
some extent, and cases have been known where animals have 
actually died of the inflammation produced. Some idea of the 
extent of the plague may be realised from the statement that an 
Indian kip in the museum of the Leeds University Leather 
Department has not less than 680 warble-holes, and that almost 
equal numbers have been counted on English hides. Warbled 
hides are useless for many of the purposes to which leather is 
applied, and the aggregate annual loss on the hides alone is 



46 PRINCIPLES OF LEATHER MANUFACTURE 

estimated at not less than £500,000. Another way in which 
damage is done is by the state of terror which the attack of the 
fly causes in herds of cattle, causing them to " gad " or gallop 
aimlessly about. This is the more singula^', as the laying of the 
egg produces no pain, and cattle can hardly be supposed to foresee 
its result, but the attack of the fly is very persistent, and cattle 
are sensitive to touches on the leg. Some protection is afforded 
if the cattle can get in the shade, as the fly seems only to strike 
in sunshine, and cattle also take refuge in water, where the fly 
does not pursue them, and where the lower part of the leg is 
protected. 

Deer and goats, and occasionally horses, are attacked by the 
warble fly, but sheep seem to be exempt, though they are subject 
to other ills too numerous to mention ; but reference must be 
made to two which affect the skin — cockle and colt. These can 
hardly be said to be diseases, except from the tanners' point of 
view, as they neither cause pain nor affect the health of the 
sheep, though they deteriorate the value of its skin. 

Cockle is described by Seymour- Jones ^ as "a disturbance of 
the pelt structure, resembling a hard pimple or boil of a. marked 
and well-defined appearance, dark in colour, especially after 
liming, and turning to a deep brownish-black in the tan liquors. 
The area covered varies in different skins according to given 
conditions. The markings range in regular waves or ridges 
from the spine outwards, commencing in the region of the neck 
and shoulder {i.e. the heart), and sometimes covering the whole 
skin, but frequently ceasing with the ribs." (The name is derived 
from the shell-like form of the markings.) 

" After depilation the cockle assumes a yellow colour, which 
deepens in tint as the operations succeed each other, and may 
easily be taken for a species of gristle." 

" In mild cases the wet-work treatment wiU remove the earlier 
forms, but when the cockle is of long standing, no treatment at 
present applied will remove them." 

Cockle begins to appear about December, and increases until 
the sheep is shorn, when it entirely disappears. Seymour- Jones 
had the right-hand side of iifteen sheep shorn, and when 
slaughtered three days later, the cockle had almost disappeared 
on the shorn side, though it was marked on the other. These 
sheep were also subjected to different feeding: five fed with 
extra oilcake and dry foods displayed cockle in its worst form, 
five with oilcake alternated with mangolds showed it in medium 

^ Seymour-Jones, The Sheep and its Skin, Leather Trades Publishing 
Co., 1913. 



ORIGIN AND CURING OF HIDES AND SKINS 47 

degree, while five fed with roots and moist food showed it only 
mildly. It is evident, therefore, that cockle is much influenced 
by the feeding, and is much increased by the intensive feeding 
now generally adopted to prepare sheep earlier for the market. 

A fatty scurfy deposit appears on the cockly skin under the 
wool. At Seymour- Jones' request the writer made what chemical 
examination he could of a very small sample, unfortunately 
insufficient for any complete analysis. The fatty portion appears 
to consist of a mixture of glyceride fats partially oxidised, with 
some cholesterol fat possibly arising from the sebaceous glands 
and unsaponifiable by caustic potash, though it is saponified, 
or at least emulsified, by steapsin, one of the pancreatic ferments. 
Seymour- Jones believes the disease to be connected with the 
extra demand for fat made by the rapid growth of the wool during 
the winter season, though the exact mechanism of the process is 
not very clear. 

" Colt " or " dead-fat " in sheep-skins is another trouble very 
possibly increased by the present intensive feeding, and is an 
accumulation of fat in the skins, which Seymour- Jones attributes 
to an adipose degeneration of the fat-cells, probably leading to 
hydrolysis of the fat and crystallisation of the fatty acids set 
free. 




Fig. 8a. — Hypoderma lineata. 



CHAPTER VII 

STRUCTURE AND GROWTH OF SKIN 

Although, at first sight, the skins of different animals appear 
to have httle in common, a closer examination shows that all 
the Mammalia possess skins which have the same general struc- 
ture, and thus a general anatomical description of the skin of an 
ox applies almost equally to that of a sheep, goat, or calf, though 
on account of the difference in texture and thickness the practical 
uses of these various materials may differ widely. The skins of 
lizards, alligators, fishes, and serpents differ from those of the 
higher animals, chiefly in having considerable modifications in 
the epidermis, so that it becomes harder and forms " scales," 
and the arrangement of the fibres presents considerable differ- 
ence. In many fish-skins, for instance, the fibres are in successive 
layers, at right angles to each other and diagonal to the skin, 
like the threads in a " Palmer " tyre, but not interlaced. 

In its natural condition the skin is not merely a covering for 
the animal, but at the same time an organ of sense, secretion 
and excretion, and hence its structure is somewhat complicated. 
It consists of two principal layers, the epidermis [epithelium, 
cuticle) and the corium [derma, cutis). These are totally distinct, 
not only in structure and functions, but in their origin. In the 
egg of a bird and the ov^tm of a higher animal the living germ 
consists of a single cell, which, as soon as fertilised, begins to 
multiply by repeated division [cp. Chapter III.) . The mass of cells 
thus formed early differentiates into three distinct layers, from 
the upper of which the epidermis arises, the corium, together with 
the bones, muscles, and cartilages, is derived from the middle one, 
and the lower furnishes the epithelial lining of the internal organs. 
This distinction of origin corresponds to a wide difference of both 
anatomical and chemical characteristics. The upper and lower 
layers are formed of cells, of which the walls mostly consist of 
keratins, and which multiply by division, while the structures 
derived from the middle layer consist largely of connective tissue, 
and yield gelatine and allied substances by boiling, and though 
produced by cells, are not themselves cellular. 

A diagrammatic section of skin is shown in fig. 9, but for the 
actual microscopic appearance of its various parts the reader 

48 



STRUCTURE AND GROWTH OF SKIN 49 

is referred to the very beautiful photomicrographs given by Mr 
A. Seym our- Jones in his series of articles on the " Physiology 
of Skin " in the Journ. of the Society of Leather Trades Chemists 
(beginning in September 1917, and continued at intervals to the 




Fig. 9. — Vertical section of calf-skin, magnified about 50 diameters. 
a, epidermis ; b, grain or papillary layer ; c, fibrous layer of skin ; d, 
hairs ; e, fat-glands ; /, sweat-glands ; g, opening of ducts of sweat- 
glands ; h, hair-muscles. Only a small part of the coarsely fibrous part 
of the corium is shown, and it extends somewhat farther upwards than is 
shown in the drawing. 



present time), and one of which is by his permission used in 
illustrating the present chapter. 

As will be seen from fig. 9, the upper, or epidermis layer is 
very thin as compared with the corium beneath it (the whole of 
which is not included), and it is entirely removed in the process 
of depilation, but it is yet of importance to the tanner, since 
the hair and fat- and sweat-glands, although rooted in the 
corium, are all products of the epidermis, and their complete 
removal depends on its chemical and anatomical character, and 
its difference from that of the corium. 

4 




50 PRINCIPLES OF LEATHER MANUFACTURE 

The corium ^ consists mainly of a mass of felted fibres of white 
" connective tissue," consisting of collagen, which on boiling takes 
up water, and is converted into gelatine. In its lower part, con- 
stituting the bulk of the hide or skin, these fibres are compara- 
tively coarse, and are made up of 
bundles of much finer fibrils bound 
together in some way, but in the 
thin upper layer, which the tanner 
distinguishes as the " grain," the 
texture is much finer, the bundles 
appear to be split up into their 
Fig. io. — Epidermis layer, individual fibrils, and are to a con- 
siderable extent mixed with a net- 
work of the so-called "elastic" or "yellow" fibres, which are 
insoluble in hot water, and quite different chemically from the 
" white " collagen fibres. The hair-bulbs, with their sebaceous 
glands and sheaths of epidermis cells, mostly pass completely 
through this grain-layer into the coarser tissue beneath, which 
may be well distinguished as the pars fasciculi or " bundle layer." 
The epidermis is shown in fig. 9 at a and in fig. 10, at a and h, 
more highly magnified. Its inner mucous layer h, the rete 
malpighi, which rests upon the corium c, is soft, and composed 
of living nucleated cells, which multiply by division, and form 
cell-walls of keratin. These are elongated in the deeper layers, 
and gradually become flattened as they approach the surface, 
where they dry up and form the horny layer a. This last is being 
constantly worn away, thrown off as dead scales of skin (" scurf "), 
and as constantly renewed from below by the multiplication of 
the cells. It is from this epithelial layer that the hair, as well 
as the sweat- and fat-glands, are developed. The epidermis is 
not supplied with blood- or lymph-vessels, so that its cells 
are nourished entirely from the juices of the corium on which 
it rests, and hence die as they are pushed away from it 
by the younger cells. The human epidermis, especially on the 
soles of the feet and the palmar surface of the hand, is much 
thicker than that of most domestic animals, and much mis- 
apprehension has been created by the supposition that draw- 
ings of these thicker parts represented the usual proportions 
of animal skin. Indeed in one of the older tanning manuals 

J- Corium (Lat.) or Derma (Gr.) each signify both hide and leather, and 
presumably, when used for hide, include the whole hide which is made 
into leather, and are synonymous with pelt (Fr. Cuir en tripe, Ger. Blosse) . 
Epidermis is the layer which lies upon the derma, and therefore above the 
grain. 



STRUCTURE AND GROWTH OF SKIN 51 

such a representation of human skin, originally taken from a 
drawing of the German histologist Kolliker, was boldly labelled 
" section of calfskin " ! In these thick parts of the human 
epidermis the ducts of the sweat-glands take a spiral course which 
cannot usually be noticed in the sections of animal skin, and many 
histologists have divided it into several layers, strata mucosum, 




Fig. 1 1 . — Section of calf-skin, showing flattening of epidermal 
cells. (X720.) Photograph by R. H. Marriott. 



lucidum, and corneum, which are really only successive stages 
of development and decay. The stratum lucidum, a translucent 
layer between the mucous and the horny layers, deserves a 
passing mention from its analogy with the action of the sebaceous 
glands. As the cells degenerate and dry up they become satu- 
rated with an oily matter, eleidin, which they secrete, and so 
are rendered more transparent. Similarly, the cells of the 
sebaceous glands, as they are pushed inwards and away from the 
corium tissue with which they are surrounded, and from which 
they derive their nourishment, become degenerated and 
break down, and discharge their fatty contents into the cell, 



52 PRINCIPLES OF LEATHER MANUFACTURE 

and ultimately into the hair-sheath, where it lubricates the 
hair. 

The hair is derived from the epidermis, and is contained in a 
sheath of nucleated cells, which is continuous with the epidermis 
and completely surrounds its bulb, so that it is altogether an epi- 
dermis product, though rooted deeply in the corium. The hair 




Fig. 12. — Section of goat-skin showing]isweat-ducts^andtsebaceous 
glands. Unhaired. (X84.) Photograph by R. H. Marriott. 



itself is covered with a layer of overlapping scales, like the slates 
on a roof, but of irregular form. These give it a serrated outline 
at the sides, and when strongly developed, as in wool and some 
furs, and further raised by chemical treatment, confer the pro- 
perty of felting. Within these scales, which are called the " hair 
cuticle," is a fibrous substance which forms the body of the 
hair ; and sometimes, but not always, there is also a central 
and cellular pith, which under the microscope frequently appears 
black and opaque, from the optical effect of imprisoned air. 
On boiling or long soaking in water, alcohol, or turpentine 



STRUCTURE AND GROWTH OF SKIN 53 

the air-spaces become filled with the liquid, and then appear 
transparent. 

The fibrous part of the hair is made up of long spindle-shaped 
cells, and contains the pigment which gives the hair its colour. 




Fig. 13. — Photomicrograph by Mr A. Seymour- Jones of a hair, hair 
follicle, and the erector pili muscle. 

The hair may be clearly traced downward to the root bulb and surround- 
ing the hair follicle. The attachment of the erecting muscle (p. 55) is 
shown on the left side of the hair follicle, starting at the top, and gradually 
terminating near the root bulb. Its conformation resembles the spreading 
of the oak tree at its base. Its powerful hold is well defined, and the reader 
will appreciate, therefrom, its action. The method of attachment to the 
grain membrane of this muscle is similar to that of the hair follicle. 
Between the points of attachment the muscle is shown reduced in thick- 
ness, but this reduction is not maintained beyond the point shown in 
the figure — that is, the thickness remains even throughout until it again 
meets the attaching end, when it spreads out to secure a strong 
foundation grip. 



54 PRINCIPLES OF LEATHER MANUFACTURE 




These cells are easily seen under the microscope if hair or wool is 
broken up by treatment with moderately strong sulphuric acid. 
The hair of the deer differs from that of most other animals in 
being almost wholly formed of polygonal cells, which, in white 
hairs, are usually filled with air. In all dark hairs both the hair 
and sheath are strongly pigmented, but the hair is much the more 
so, and hence the bulb has usually a distinct dark form. The 

dark-haired portions of a hide 
from which the hair has been 
removed by liming still remain 
coloured by the pigmented cells 
of the hair-sheaths, which can 
only be completely removed by 
bating and scudding. 

Just below the grain-layer the 
ducts of the sebaceous or fat- 
glands pass into the hair-sheath 
and secrete an oily matter to 
lubricate the hair. The glands 
themselves are formed of large 
nucleated cells arranged some- 
what like a bunch of grapes, the 
upper and more central ones 
being highly charged with fatty 
matter. Their appearance is shown in fig. 14. A good deal of 
the pasty substance worked out in unhairing (yellow on a 
white hide) consists of these sebaceous glands, more or less 
broken down by liming, but still recognisable by the microscope. 
The base of the hair is a bulb, enclosing the hair papilla h (fig. 
15), which is a projecting knob of the corium, and which by 
means of the blood-vessels contained in it supplies nourishment 
to the hair. The hair-bulb is composed of round soft cells, 
which multiply rapidly by division, and pressing upwards through 
the hair-sheath become hardened, thus increasing the length of 
the hair. 

The cells outside the bulb, shown at / in fig. 15, pass upwards 
as they grow, and form a coating around the hair, known as the 
" inner root -sheath." 

In the larger text-books of histology a good deal more detail is 
given of the structure of hair and its sheath, which is omitted 
here as unimportant for the tanner. 

In embryonic development a small knob of cells forms on the 
under side of the epidermis, over a knot of capillary blood-vessels 
in the corium, and enlarges and sinks deeper into the latter, while 



Fig. 14.— a, sebaceous gland ; 
b, hair ; c, erector muscle. 

(X200.) 



STRUCTURE AND GROWTH OF SKIN 55 



the root-bulb of the young hair is formed within it, surrounding 

the capillaries from which it derives nourishment, and which 

form the hair-papilla, fig. 15. In the renewal of hair in the 

adult animal the process is very similar. The bulb of the old hair 

withers and the hair falls out, and in the 

meantime a thickening takes place in the 

epidermal coating of the lower part of the 

sheath, and the young hair is formed below, 

and usually to one side of the old one, 

growing into the sheath, and taking the 

place of the old hair. This is one cause 

of the difficulty of removing ground-hairs 

in the process of unhairing, since they are 

not only short, but deeper seated than the 

old ones. 

The process of development of the sudori- 
ferous or sweat-glands is very similar to 
that of the hairs. They consist of more 
or less convoluted tubes surrounded with 
walls formed of longitudinal fibres of con- 
nective tissue of the corium, lined with a 
single layer of large nucleated cells, which 
secrete the perspiration. The ducts, which 
are exceedingly narrow, and with walls of 
nucleated cells like those of the outer hair- Fig. 15. 
sheaths, sometimes open directly through 
the epidermis, but more frequently into 
the orifice of a hair-sheath, just at the 
surface of the skin. Each hair is provided 
with a slanting muscle called the arrector 
or erector pili (see fig. 17), which is con- 
tracted by cold or fear, and causes the 
hair to " bristle " or stand on end ; by forcing up the attached 
skin, it produces the effect known as " goose-skin." The muscle, 
which is of the unstriped or involuntary kind,^ passes from near 
the hair-bulb to the epidermis, and just under the sebaceous 
glands, which it compresses when it contracts, thus forcing out 
the oily matter. 

Beside the hair, and hair-sheaths, and the sebaceous and 
sudoriferous glands, the epidermis layer produces other structures 

^ The muscles which are under the control of the will, and known as 
" voluntary muscles," are composed of thin plates, giving them a finely 
striped appearance under the microscope, while those controlled by the 
sympathetic nervous system appear simply fibrous. See fig. 21. 




-a, hair; &,hair 
cuticle ; c, inner root- 
sheath; ^, outer root- 
sheath ; e, dermic 
coat of hair-sheath ; 
/, origin of inner 
sheath ; g, bulb ; h, 
hair-papilla. 




56 PRINCIPLES OF LEATHER MANUFACTURE 

of a horny character, including horns, hoofs, claws, and finger- 
nails ; which both chemically and anatomically are analogous 
to exaggerated hairs, such as the quills of the porcupine. The 
feathers of birds, and probably the scales of fishes and reptiles, 

have a similar origin. None of 
these epidermal structures are 
soluble in hot water, or capable 
of producing gelatine or glue. 
The whole of the hair-sheath and 
its glands is enclosed in a coating 
of elastic and connective-tissue 
fibres, which are supplied with 
nerves and blood-vessels, and form 
Fig. 1 6. — Development of part of the COrium. 

young hair. The epidermis, together with the 

hairs, is separated from the corium 
by an extremely fine membrane or surface called the hyaline 
(hyaline = glassy). This has been the subject of much mis- 
conception, for it is so extremely thin that many histologists 
have doubted even its existence, and have failed to demonstrate 
it microscopically, and it has been erroneously identified with 
the upper part of the grain-layer, which, when dry and very thin, 
as in the " fly -wing " skiver, is almost transparent. Kathreiner 
believed that he had separated it by accident in the tannery, 
and the writer had one of his preparations, but it was so trans- 
parent that it could scarcely be actually seen under the micro- 
scope, though its existence could be inferred by the specks of dirt 
and broken-down cells which adhered to it. That it has, how- 
ever, a real existence is proved by its forming the very thin buff- 
coloured surface of tanned leather, which takes dyes, as well as 
the colouring matters of the tan differently to the grain immedi- 
ately beneath it, from which it is evidently distinct. If it gets 
scraped off mechanically in the beamhouse, or destroyed bac- 
terially in the limes, the exposed portion of the skin remains nearly 
white instead of colouring, although it has been equally exposed 
to the tanning liquors. Indeed it is almost impossible to decide 
by examining the finished leather whether the damage has been 
done in the beamhouse or after the leather was fully tanned. 
Seymour-Jones believes that the hyaline is always destroyed by 
the use of strong solutions of alkaline sulphides, but such solutions 
are sometimes used in the unwooling of cheap glove-leathers, where 
the preservation of the grain-surface is very important. To avoid 
misconception, the hyaline, if referred to in the following pages, 
will be called the " grain-surface." Its quantity is so small that 



STRUCTURE AND GROWTH OF SKIN 57 

it is impossible to study it chemically except upon the skin, and 
it is not known whether it originates from the corium or the 
epidermis. It is possibly identical with the " basal membrane," 
which can be demonstrated in the hair-bulbs. Seymour- Jones 




Fig. 17. — Section of calf-skin, showing arrector pili muscle and develop- 
ment of hairs. (x8o.) Photograph by R. H. Marriott. 



suggests that it is a more or less artificial result of liming, and 
derived from the bases of the cells of the mucous layer. It is 
not identical with elastin, since it is not digested by the trypsin 
ferments. A clear layer of about o'oi mm. in thickness is very 
well shown in the puered skin on fig. 42, p. 226, but whether 
this is really the hyaline is uncertain. 

The structure of the corium is quite different from that of the 



58 PRINCIPLES OF LEATHER MANUFACTURE 

epidermis, which has just been described, as it is principally 
composed of interlacing bundles of white fibres, of the kind 
known as " connective tissue " (see fig. i8) ; these are composed 
of fibrils of extreme fineness, supposed to be cemented together 
by a substance somewhat more soluble than the fibres them- 
selves. The fibres are not themselves living cells, but are appa- 
rently produced by narrow spindle-shaped cells lying against them. 




Fig. 1 8. — Connective-tissue fibres. (Ranvier.) 



and possibly travelling up and down the fibre. Rows of some- 
what similar cells exist between the fibres of sinews, which are 
also of connective tissue, though much more compact than that of 
skin, and in parallel bundles instead of felted together. Inter- 
woven with the white fibres of the grain-layer is a network of the 
so-called " yellow " or " elastic " fibres. These are chemically 
very different from the white fibres, which consist of collagen, 
and when boiled take up water and become converted into glue 
or gelatines, while the elastic fibres are insoluble in boiling water, 
and do not appear to combine with tannin. They are also 
digested by the trypsin ferments, which the white fibres are not. 



STRUCTURE AND GROWTH OF SKIN 59 

a matter of cardinal importance in the puering process which will 
be further referred to. The elastic fibres form an actual net- 
work, dividing and reuniting, which the white fibres, however 
interwoven, never seem to do, but always keep their individual 
identity. When broken, the elastic fibres curl up and contract, 
and often take spiral forms. They are absent through most of 




Fig. iq. — Section of ox-hide showing elastic fibres. ( x loo.) 
Photograph by R. H. Marriott. 



the fibrous part of the corium, but become much more abundant 
as it approaches the flesh. 

The outermost layer of the corium, which has just been referred 
to as the " grain-layer," which is just beneath the epidermis, 
is exceedingly close and compact, the fibre-bundles that run into 
it being separated into their elementary fibrils, which are so 
interlaced that they can scarcely be recognised. This is the pars 
papillaris, and forms the lighter-coloured layer, called (together 
with its very fine outer coating) the " grain " of leather. It is 
just below this part that the sebaceous glands are embedded, 
while the hair-roots and sweat-glands pass through more deeply 
into the coarser fibred part below. 

The bundles of fibrils are supposed to be held together and 



6o PRINCIPLES OF LEATHER MANUFACTURE 

the spaces between them filled by some less organised and more 
soluble substance often called " cement-substance " (Ger. 
Kittsuhstanz) , of which the nature is unknown, but which seems 
to be removed by liming. Reimer thought he had separated it, 
and called it " coriin," but his product was later shown to be 
merely due to the solution of the hide-fibres themselves. Sey- 
mour-Jones {J.S.L.T.C., 1918, p. 36) speaks of the fibre-bundles 
as being " united by a soft filamentous substance of considerable 
tenacity and elasticity, known as areolar tissue." This is quite 
possible, but " areolar tissue " is only a somewhat old name for 
the looser and more open sort of connective tissue, such as is 
found below the skin, and surrounds man}^ organs of the body. 
It was originally supposed to be cellular, as the name implies, 
but is now known to be merely a network of white connective- 
tissue fibres like the corium, and only contains the flattened 
and elongated cells which are also found in the skin, and which 
apparently produce the fibres. 

The fibre-bundles in some connective tissues are encircled and 
tied together by rings or spirals of elastic fibre (see fig. 18), and 
some histologists have supposed that these were the contracted 
remains of very thin sheaths of elastin with which the fibre- 
bundles were originally surrounded, but which have been torn 
and ruptured by the swelling of the fibrils under the influence 
of acids or alkalies ; and though such sheaths have not been 
demonstrated, the theory is not altogether improbable.^ They 
seem to be absent in most of the fibre-bundles of the skin. 

As to the origin of the fibres themselves, they are not living 
cells with protoplasm and nuclei, and therefore cannot grow by 
division like the cells of the epidermis ; but lying between and 
against the fibres are many flattened and elongated cells which 
apparently produce the fibres, which may therefore be con- 
sidered as cell-products rather than actually living tissues. 
Nothing is known of the origin of the cells, which are possibly 
modified migratory cells, which latter are probably identical 
with white blood-corpuscles, lymph- and saliva-cells and the 
like, and are found not only in the liquid ducts, but wander- 
ing through the tissues, which they are able to do from their 
amoeboid character and absence of cell-walls. They apparently 
play a large and varied part in the animal economy, devouring 
injurious bacteria and effete substance of all kinds, and conveying 
it to places where it is required for building up new tissues ; and 
they are supposed to adapt themselves to all sorts of special uses. 

The pars papillaris, or grain-layer, receives its name from 
1 Seymour-Jones has spoken of these as " fibre-sarcolemma." 



STRUCTURE AND GROWTH OF SKIN 6i 

the small projections or papillcB, with which its outer surface 
is studded, and which form the characteristic grain-pattern 
of the various kinds of skin ^ (see iig. 9) , and which con- 
tain the nervous ganglia which are the organs of the sense of 
touch. 

The study of the structure of the grain, and especially of 
the arrangement of the hair-pores, is very important, as it 
is usually the readiest means of identifying the kind of 
skin of which a leather is made, which in finished skins with 
artificially printed grain is often very difficult. (Frontispiece.) 
The examination is facilitated by wetting and stretching the 
skin, and by the use of a good lens, or a low power of the 
microscope. 2 

It has been noted above that the sebaceous glands of the hairs 
are immediately below the grain-layer, while the hair-bulbs 
and sweat-glands are somewhat deeper, and frequently associ- 
ated with large numbers of fat-ceUs. This is specially the case 
in sheep-skins, where in both cases the oily matter is so abundant 
that these regions haVe been described as distinct layers, and 
spoken of as " sebaceous " and " adipose " layers. Though it 
is convenient to retain these names, neither is strictly appropriate, 
as Lat. sebum is tallow, while the fatty matter of the glands is 
mainly a liquid wax, containing no glycerine and much unsaponi- 
fiable matter, and in a purified form constituting lanoline, while 
the contents of the ceUs of the hair-bulb layer is a soft tallow, 
glycerides of oleic and stearic acids ; and the term " adipose 
layer " is objectionable as liable to confusion with the panniculus 
adiposus underlying the corium, which also contains large 
quantities of similar fat-cells. It may be noted that these cells, 
which are frequent in loose connective tissue throughout the 
body, are reaUy living cells with nuclei and protoplasm, and the 
fat cannot be set free and worked out or expressed till the cell- 
walls are broken down by liming or some other means. They 
are shown in fig. 20. 

^ It will be noted that the word " grain " is used by the tanner in at 
least three different senses, which are productive of much confusion. The 
extremely thin hyaline forms a natural glaze to the skin, and might well 
be spoken of as such ; the form and arrangement of the papillce and hair- 
pores might be called the " pattern " of the grain, leaving the use of the 
word " grain " or " grain-layer " itself restricted to the pars papillaris. 

^ Under the microscope the skin is of course lighted from above by 
direct light from a window, or by that of a lamp concentrated by a " bulls- 
eye " condenser. The reversal of the image in the microscope often causes 
a pseudoscopic effect very puzzling to the beginner, prominences appearing 
as hollows, and vice versa, till the real direction of the lighting is considered. 



62 PRINCIPLES OF LEATHER MANUFACTURE 

It has been remarked that both these fatty layers are specially 
prominent in sheep-skins, and each, when the sebaceous glands 
and hair-bulbs have been removed at depilation, constitute planes 
of weakness in the skins. The grain not unfrequently peels at 
the sebaceous layer, and it is through this that the cut is taken 
in splitting " fly-wing " skivers, while for ordinary skivers it 
is through, or just above, the region of the hair-bulbs ; and the 




Fig. 20. — Fat-cells in connective tissue, a, fat-globule ; p, protoplasm ; 
n, nucleus ; m. cell-wall. (Ranvier.) 



fatty layer on the surface of the " flesh-split " is removed by 
skiving before chamoising. In splitting hides in the United 
States the same loose fatty layer is removed from the flesh-split 
by a second thin cut, the split being turned flesh side up in the 
machine, so as to make the substance of even thickness from the 
flesh surface, which is to be waxed or otherwise finished. In 
old days, before the splitting machine was invented, grains were 
frequently torn from the flesh at this point with a little assistance 
of the hand-knife. 

As stated above, the surface of skin which is next to the flesh 



STRUCTURE AND GROWTH OF SKIN 63 



is firmer than that in the centre, and contains a good deal of 
elastic fibre, and as the fibres run nearly parallel with the surface, 
it has a more or less membranous character. The skin is united 
to the body of the animal by a network of areolar connective 
tissue [panniculus adiposus), which is frequently full of fat-cells, 
and is then called adipose tissue. This constitutes the whitish 
layer which is removed, together with portions of actual flesh, 
in the operation of " fleshing." If a 
minute portion of adipose tissue be 
examined microscopically, it will appear 
to consist of a mere mass of fat-globules 
entangled in connective tissue. If, how- 
ever, it be stained with carmine or log- 
wood it may be at once observed that 
each globule is contained in a cell, of 
which the. nucleated protoplasm, by which 
the fat was secreted, is pressed closely 
against the wall (fig. 20). 

Many animals (ox, horse, etc.) possess a 
thin layer of voluntary muscle (red flesh, 
panniculus carnosus) spread over the inner 
side of the skin, and used for twitching to 
drive off flies. In rough fleshing this is 
sometimes left on, and may be a cause of 
dark flesh in sole leather. Even in the 
finished leather its striped structure may 
be detected microscopically (fig. 21). 

Besides the connective- tissue fibres, the 
skin contains a varying proportion of fine 
yellow " elastic " fibres, especially in the 
grain-layer. If a thin section of hide be soaked for a few minutes 
in a mixture of equal parts of water, glycerine, and strong acetic 
acid, and then examined under the microscope, the white con- 
nective-tissue fibres become swollen and transparent, {cp. pp. 
223-227 and figs. 40-43), and the yellow " elastic" fibres may 
be seen, as they are scarcely affected by the acid. They are, 
however, much better seen when stained blue with fuchsin and 
Weigert's stain. The hair-bulbs and sweat- and fat-glands are 
also rendered distinctly visible by treatment with acid glycerine. 
On the other hand, the white gelatinous fibres are most easily 
seen by examining the section in a strong solution of common 
salt, slightly acidified with acetic acid, or in one of ammonium 
sulphate ; or by staining with some aniline dyes such as safranine. 
Sections are most readily cut for these purposes by the use of the 




Fig. 21. — Striped, or 
voluntary muscular 
fibre. (Ranvier.) 



64 PRINCIPLES OF LEATHER MANUFACTURE 

freezing microtome, or after previous hardening in alcohol. For 
further details see L.I.L.B., p. 391, and the articles of Seymour- 
Jones in the J.S.L.T.C., 1917, et seq. 

Ordinarily in the production of leather only the corium, or 
fibrous part of the skin, is used, and in order to obtain it in a suit- 
able condition for the various tanning processes, the hair or wool, 
together with the epidermis, must be completely removed' without 
damaging the skin itself ; and especial care must be taken that 
the grain-surface or portion next to the epidermis does not suffer 
any injury during the treatment. All the methods employed 
depend upon the fact that the epidermis cells, especially the soft- 
growing ones next to the corium, and those of the epidermis layer 
which surround the hair-roots, are more easily destroyed than the 
corium itself, owing to their different chemical character. The 
" unhairing " process consists essentially in breaking down these 
cells by chemical or putrefactive agents, and removing the hair 
together with the rest of the epidermis by mechanical means. 
Of the various substances which may be used for this purpose 
lime is one of the most convenient, as its solubility in water is 
so slight, that a solution of such a strength as rapidly to injure 
the hide cannot be made, and lime, being divalent, causes only 
half the swelling of monovalent alkalies. Caustic alkalies^ on the 
other hand, are much more soluble, and unless care be taken to use 
only the proper quantity, a dangerously strong solution may be 
made with consequent damage to the skin. The addition of small 
amounts of sulphides to the lime solution accelerates the unhairing 
owing to their special solvent action on the epidermis structures, 
and also in the case of alkaline sulphides, by the caustic alkali 
which is produced by their reaction with the lime. Even if 
used alone, strong solutions of alkaline sulphides rapidly 
destroy both hair and epidermis, converting them into a mass 
which may be swept off the skin like wet pulp, and yet they 
have practically no injurious action on the true skin. Rohm 
has patented the use of the pancreatic ferments (trypsin, etc.) 
for unhairing. 

In the " sweating " process the epidermis cells are broken 
down by putrefactive organisms and their products, especi- 
ally the tryptic ferments, so that the hair becomes loose, 
and may then be either rubbed or scraped off. Ammonia, 
which is produced during the putrefaction, has also an 
important solvent action, and its presence doubtless tends 
to quicken the processes both of unhairing and of de- 
struction. 

To obtain useful knowledge of the structure of any particular 



STRUCTURE AND GROWTH OF SKIN 65 

skin it is not necessary to have a very elaborate or expensive 
microscope, and it is quite possible to obtain much information 
merely by the use of a good pocket lens, as, for instance, in the 
examination of various forms of " grain " and the embossing of 
one skin to imitate another. 



CHAPTER VIII 

WATER AS USED IN THE TANNERY 

Of all the materials employed in tanning, none is of more indis- 
pensable importance than water, and its quality has undoubtedly 
great influence on tanning, though it is constantly blamed for 
faults and troubles which are really due to the mistakes of the 
tanner. 

Water is chiefly used in tanneries for soaking and washing 
hides and skins, for making the limes, the bates, and the tanning 
liquors, for steam boilers, and in dyeing. For all these purposes 
it should be as free as possible from impurities, but since water is 
the most universal solvent in Nature, it is never found pure, but 
always contains mineral matter derived from the rocks and 
soil through which it has flowed, as well as organic impurities 
from decaying animal and vegetable matter. Associated with 
the latter are usually living organisms of putrefaction {bacteria), 
which may affect the quality of the water for tanning even 
more seriously than the mineral impurities. The purest natural 
waters are those which have flowed only over hard sandstones 
and volcanic rocks, while limestone dissolves freely in the carbonic 
acid of rain-water. Water sufficiently pure "for laboratory use 
can only be obtained by distiUation. The steam-water from 
heating pipes usually contains large quantities of dissolved 
iron, and often also volatile organic matters from the oil, etc., 
which finds its way into the boiler. It may sometimes be made 
fit for use by boiling (which precipitates the ferrous carbonate 
present), and subsequent settling or filtration. The use of steam- 
water containing iron is a frequent source of stains and discolora- 
tions in the tannery which more than counterbalances the 
advantage of its softness. 

The " hardness " of natural waters is mostly due to the salts of 
lime and magnesia which they contain, which precipitate soap in 
the form of insoluble stearates and oleates, which are useless for 
washing. It is commonly estimated by determining the amount 
of a standard alcoholic soap solution which must be added in 
order to produce a permanent froth on shaking. Theoretically 
about 12 parts of soap (sodium stearate or oleate) are destroyed 
by I part of calcium carbonate or an equivalent quantity of 

66 



WATER AS USED IN THE TANNERY 67 

other lime salts, with formation of insoluble lime soaps (calcium 
stearate or oleate). Really, the reaction is much more compli- 
cated, owing to the dissociation of the soap into free alkali and 
acid-salts on solution in water. Teed ^ estimates that | to |- more 
is required than the theoretical quantity, and more in hot water 
than cold. This uncertainty is partially overcome by testing the 
soap solution against a known solution of calcium chloride. The 
presence of magnesia also complicates the test and leads to 
discrepant results. 

The methods of determining hardness originated by Hehner 
(see L.I.L.B., p. 19, and L.C.P.B., p. 16) are simpler and more 
accurate than the soap test, and are to be preferred, except for 
direct determination of the suitability of a water for scouring 
with soap. " Degrees " of hardness in England are calculated 
as parts of CaCOg per 100,000, or sometimes as grains per gallon 
(70,000 grains). German degrees are parts of CaO in 100,000, 
and consequently 56 German = 100 English. 

Hardness is of two kinds, " temporary " and " permanent " ; 
the former being removed by boiling, while the latter is not so 
removed. 

Temporary hardness consists of the carbonates of alkaline 
earths held in solution by an excess of carbonic acid. Lime com- 
bines with I molecule of carbon dioxide to form the ordinary 
normal carbonate (chalk), which is practically insoluble in water. 
When, however, excess of carbonic acid is present, hydric calcic 
carbonate (bicarbonate) which is fairly soluble is produced. 
This is easily demonstrated by passing carbon dioxide into 
somewhat diluted lime-water, which at first becomes turbid from 
precipitated chalk, but soon clears by formation of soluble 
hydric carbonate. If the solution be now boiled, the hydric 
carbonate is decomposed, and the excess of carbonic acid is 
driven off as CO2, and the chalk again precipitated. The 
reactions are represented by the following equations : — 

Ca(OH)2+C02-CaC03-HOH2. (i) 

CaC03-f-C02-FOH2= I ^^^^^ (2) 

Magnesia forms soluble double carbonates in a similar manner, 
but on continued boiling gradually loses the whole of its carbonic 
acid, and is precipitated as magnesium hydrate, Mg(0H)2. 

One of the most important reactions in connection with 
temporary hardness is that caused by the addition of calcium 
hydrate (slaked lime), which forms the basis of Clark's soften- 

^ Journ. Soc. Chem. Ind., 1889, p. 256. Cp. also Allen, ibid., 1888, p. 795. 



68 PRINCIPLES OF LEATHER MANUFACTURE 

ing process. When an equivalent amount of lime is added 
to a solution of hydric calcic carbonate, it displaces the water 
of the " half-bound " carbonic acid, forming a second molecule 
of calcium carbonate, which is precipitated together with that 
originally present, as is represented in the following equation : — 

U^ol } +Ca(OH)2 = 2CaC03+20H2. (3) 

Hydric magnesium carbonate is also precipitated by lime, but 
the reaction is somewhat different, the magnesia being removed 
as hydrate as follows : — 

It will be noted that n2 equivalents of lime are required to 
precipitate i of magnesia. Two molecules of sodium hydrate 
(NaOH) or potassium hydrate (KOH) may be substituted for 
I of Ca(0H)2 with similar results, and in some cases it is prac- 
tically advantageous to use the former, as the sodium carbonate 
formed in precipitating the temporary hardness reacts again 
on the permanent, throwing down the lime and magnesia as 
carbonates. (See p. 78.) 

The use of lime for softening temporary hard waters was 
originally proposed by Thomas Henry, F.R.S., of Manchester, 
but was first applied as a practical process by Clark, who, after 
adding the requisite quantity of lime to the water in a mixing 
vat, allowed it to stand in a large tank to clear by subsidence, 
the precipitated carbonate of lime taking from six to twelve hours 
to settle. The process in its original form is a perfectly satis- 
factory one, except for the capacious settling tanks which are 
required, which in some cases are inconvenient and expensive. 
Messrs Archbutt and Deeley ^ patented a modification of the 
Clark process, by which the time of subsidence is much shortened, 
and according to which the precipitated carbonate of lime of 
previous operations is allowed to remain in the tank, and the 
fresh charge of water and lime is mixed up with it by means of 
steam-injectors, which blow in a current of air through perforated 
pipes at the bottom of the tank, and at the same time very 
slightly warm the water. The action goes on much more rapidly 
at a slightly raised temperature than in the cold ; and rather 
curiously, the stirred up precipitate, instead of increasing the 
time of clearing, settles rapidly and carries down with it that 
formed in the new operation. The process is particularly suit- 
able for treating waters containing magnesia, from which a 
^ Journ. Soc. Chem. Ind., 1891, p. 511. 



WATER AS USED IN THE TANNERY 69 

compound of lime and magnesia is apt to be precipitated in a 
colloid form, which chokes filter-cloths and will not readily settle. 




After softening, the water is usually " carbonated " by passing 
the gases produced by burning coke into the floating exit-pipe 



^o PRINCIPLES OF LEATHER MANUFACTURE 



through which it falls, in order to retain any remaining traces of 
carbonates of lime and magnesia in a soluble form, and prevent 
their subsequent precipitation in the pipes. The apparatus is 
made by Messrs Mather & Piatt of Manchester, and its arrange- 
ment is shown in figs. 22 and 23. 

Several modifications of the Clark process have been intro- 
duced, in which the precipitation is carried on continuously 

Server txL Arrarvgerrhent of AppartLlixs 
for softer tin <j 8,000 gallorts per- hvicr. 



Tr artsve rse^ Sec ti ort-. 



water 
level 




Fig, 23. 

instead of intermittently. The most important of these is the 
Porter-Clark, in which one portion of the water to be softened 
flows through an agitator containing excess of hme, with which 
it forms saturated hme-water, which is passed slowly up a 
cylinder, where it deposits the excess of suspended lime. The 
clear lime-water so produced is mixed with a fresh portion of 
the water to be softened in a second cyHnder, also provided with 
an agitator, the proportion of the two liquids being regulated 
by cocks. The carbonate of lime is at once precipitated, and is 
removed by passage through a filter-press. This process has 
long been in successful operation on a considerable scale at 
Messrs Hodgsons' tannery at Beverley. 

Several other forms of filter have also been employed with 



WATER AS USED IN THE TANNERY 



71 



success, and also methods in which the treated water traverses 
tanks with sloping partitions, on which the carbonate of lime is 
deposited. The latter plan was originally patented in France 



M tS 




Fig. 24. 



by Gaillet-Huet, and has been introduced into England by 
Stanhope. 

A very good automatic apparatus is now made by Messrs 
Royles Ltd. of Irlam, near Manchester, of which a section is shown 
in fig. 24. 

" In this apparatus the bicarbonate of lime is precipitated 
by hydrate of lime (lime water), and the sulphate of lime by 
soda. The softened water is then filtered and discharged. The 
apparatus acts entirely automatically ; all that is necessary is 
to supply the lime and soda daily, and follow the very simple 
and clear directions supplied with each apparatus. 



72 PRINCIPLES OF LEATHER MANUFACTURE 

" The apparatus consists essentially of an automatic lime 
saturator and decanter B, a soda chamber D, a water-distributing 
tank E with a lime-slaking division A and soda-mixing division 
C, a reaction chamber F, and an automatic self-cleansing filter K. 

" The lime saturator B requires no stirring contrivance, and 
lixiviates the lime completely ; and owing to its upward widening 
shape, it turns the saturated lime water into a clear fluid. 

" By a constantly uniform water supply through the micrometer 
valve 3 from the distributing tank E, which is conducted down- 
wards through the central pipe O, the lime paste at the bottom 
is stirred up and thoroughly impregnated, and rises, at first 
partly with the water, until the rapidity of the rising water 
diminishes so much, owing to the upward widening shape of the 
saturator, that the undissolved lime particles are no longer able 
to follow, so that the saturated lime water, clarified, then leaves 
the saturator B at the top, and is carried into the reaction chamber 
F by way of the mixing pipe E'. 

" Lime dissolves in a definite proportion in water until the latter 
is saturated. This property renders it possible to continually 
keep a uniform stream of saturated lime water supplied as re- 
quired for the precipitation of bicarbonates of lime and magnesia. 
This uniform addition of lime water is so important that on it 
substantially depends the degree of purification of the water. 
Neither in the form of powder nor milk of lime can lime be added 
in so constantly uniform quantity. 

"The soda apparatus D acts in an equally simple and safe manner. 
Whereas lime dissolves only in a definite proportion, soda is much 
more soluble. A quantity of soda that will suffice for one or more 
da5^s is dissolved in the chamber C and let down into D through 
cock Q. The action of the soda apparatus is based on the fact 
of the soda solution having a greater specific gravity than water, 
and thus the water flowing from the distributing tank E through 
the small micrometer valve i — which is adjusted in accordance 
with the amount of soda required — into the soda chamber D, 
remains always on the surface of the soda solution (no mixing 
occurs) and displaces the same, it being carried through the small 
pipe from the bottom upwards, and into the mixing pipe E', 
and finally into the reaction chamber F." 

Mr E. Munro Payne has patented the use of sodium phosphate 
for water-softening, and it is no doubt an excellent precipitant for 
lime, both as temporary and permanent hardness, but its price 
prevents it taking the place of lime and sodium carbonate on any 
considerable scale. 

A comparatively recent process, called the " Permutit," is of 



WATER AS USED IN THE TANNERY 73 

much greater interest, both practically and scientifically. The 
following details have been kindly furnished by Mr A. Glover 
of the Research Department of the Wholesale Co-operative 
Society in Manchester, who has practical experience in its 
working : — 

" In accordance with your request, I take the liberty to send 
on a few notes on the Permutit process of water softening which 
I trust will be of use to you. I have arranged the notes under 
the following heads : — 

(i) Historj^. 

(2) Composition of Permutit. 

(3) Reactions. 

(4) Advantages. 

(5) Disadvantages. 

" (i) In recent time it has been shown that naturally soft waters 
of certain districts were originally hard, but after percolation 
through strata containing minerals known as " zeolites " gave 
soft waters. The " zeolite " is the generic name of a group of 
hydrated double silicates. The bases found in chemical com- 
bination with these silicates are principally magnesium, sodium, 
calcium, and aluminium. These zeolites exchange bases chemi- 
cally, but suffer no physical change. 

" (2) Hence the application of this idea. Permutit is a patent 
name of a synthetic compound whose chemical composition 
may be represented as follows : — 

2Si02, AI2O3. Na^O, 6H2O. 
It is a porous material, and appears like ground quartz. 

" (3) Assuming Permutit to be represented during the reactions 
as NagPm, and since Permutit gives only a base exchange during 
water softening, the following equations will explain the process 
of water softening : — 

Ca(HC03)2+Na2Pm->CaPm + 2NaHC03 
and 

Mg(HC03)2+Na2Pm-^MgPm + 2NaHC03. 

Hence the temporary hardness is destroyed, and sodium bicar- 
bonate is left in solution. In time the maximum exchange 
has taken place, and the Permutit becomes inactive. To re- 
generate the Permutit a 10 per cent, solution of common salt is 
run into the plant. Regeneration takes place as follows : — 

CaPm + 2NaCl->Na2Pm + CaClg 
and 

MgPm + 2NaCl-»Na2pm + MgClg. 



74 PRINCIPLES OF LEATHER MANUFACTURE 

The plant is now washed out, and the hard water supply is turned 
on as before. 

" Permanent Hardness. — The following equations indicate the 
reaction : — 

NagPrn + CaSO^-^CaPm + Na^SO^. 

NaaPm +MgCl2-^MgPm +2NaCl. 

After a time regeneration with common salt solution is necessary. 
" (4) Advantages. 

{a) With certain waters it is possible to get zero hard- 
ness using this plant. 

(b) No chemicals to be measured out. 

(c) No filtration, because no precipitate is produced. 

" (5) Disadvantages. 

(a) Production of NaHCOg for tannery work. This 

may be harmful. 
(&) Not applicable for all waters. A water that is too 
hard will necessitate constant regeneration of the 
Permutit. A water containing much MgCla and 
CaCla will be difficult to control, because 

NagPm + MgCla-^ MgPm + 2NaCl, 
also 

Na2Pm + CaCl2-^CaPm + 2NaCl. 

Since common salt is required for regeneration of the Permutit, 
I think that an equilibrium of the reaction must take place as 

NagPm + CaCl2'2CaPm +2NaCl. 

Hence under ordinary working conditions I do not think that all 
the permanent hardness {i.e. MgClg and CaClg), especially if it is 
high, will be removed. 

"Nevertheless, for general commercial purposes I think that 
it has many useful applications." 

So far as is yet known, from the tanner's point of view, it is 
hardly necessary to make any distinction between lime and 
magnesia, either or both of which may be considered simply as 
" hardness." A hard water probably softens dried hides more 
slowly than a purer water, though it is possible that the observed 
difference in the time required may be due in many cases to the 
lower temperature of wells from which hard water is generally 
derived. In the actual " limes " the hardness of the water can 
have no appreciable influence, though if sodium sulphide be used 
alone for unhairing, a certain waste occurs from temporary 
hardness which may render it advisable to add a little lime. It 



WATER AS USED IN THE TANNERY 75 

is in washing the hides free from lime that the influence of hard 
water is first distinctly felt. If limy goods, after unhairing, are 
placed in a water with much temporary hardness, the same 
action occurs as in Clark's water-softening process, and chalk 
is deposited in the surface of the hides, making them harsh 
and apt to " frize " or roughen the grain in " scudding." The 
common, but not wholly satisfactory, expedient is to add a little 
lime, or better, a few pailfuls of lime liquor, to the water before 
putting in the hides. The best plan is to use a properly softened 
water. Permanent hardness is not injurious in this way. 

Unfortunately it is not the grain alone which is injured by 
the use of hard water for washing the hides, but on coming into 
the liquors the precipitated bases combine with the acids and 
tannins, forming compounds which oxidise and darken when 
exposed to the air, and which are the commonest causes of 
stains and markings on all descriptions of leather. Even when 
goods are drenched or bated before tanning the injury is not 
prevented, since the weak organic acids which are capable of 
removing the lime (as such) from the hide have little effect on 
the precipitated carbonate, which can only be dissolved by the 
use of stronger acids. It must be noted that the same injurious 
effect on limed goods is produced by free carbonic acid, which 
may be present even in soft waters. 

When temporarily hard waters are employed for leaching 
tanning materials, the carbonic acid is displaced by the tannins, 
which form compounds similar to those just mentioned, which 
are incapable of tanning, and darken and discolour when exposed 
to the air. Though the amount of lime present in a liter of even 
the hardest water is very small, yet in the aggregate of thousands 
of gallons used weekly in a good-sized yard it amounts to some- 
thing very considerable, and as the molecular weight of tannins 
is very high, the quantity destroyed is many times that of the 
lime present. This loss can be prevented {a) by the addition of 
sufficient mineral acid to convert the temporary into permanent 
hardness, [b) by the use of oxalic acid, which precipitates the whole 
of the lime as oxalate, or, (c) best of all, by softening the water by 
suitable treatment before use. Each part of temporary hardness 
reckoned as CaCOg {L.I.L.B., p. 19 ; L.C.P.B., p. 17) requires 1-26 
parts of crystallised oxalic acid or 0-98 part of H2SO4, or, say, one 
part of ordinary oil of vitriol of sp. gr. 1-840 per 100,000 parts of 
water. 

As the lime and magnesia of temporarily hard water is thrown 
down by boiling, it is deposited in steam boilers as a soft pre- 
cipitate, much of which can be blown out by suitable sludging ; 



76 PRINCIPLES OF LEATHER MANUFACTURE 

but if oils or fats obtain access to the boiler, a soft, bulky, 
adherent deposit is formed, keeping the water from the plates, 
which may become red hot, and lead to collapse or explosion. 
This effect is not produced by mineral oils, which, on the con- 
trary, tend to prevent adherence of scale to the plates, and as 
suitable mineral oils are not only cheaper but much less injurious 
to the working parts of steam engines than animal or vegetable 
oils or tallow, they should always be used in preference for 
cylinder purposes. 

Water which is temporarily hard owing to calcium and 
magnesium carbonates is unsuitable for dyeing, as the carbonates 
react with basic dyes, precipitating the colour-base, and so 
rendering a part of the dye useless. Further, as this precipitate 
is deposited on the skins it causes uneven dyeing and gives rise 
to spots and streaks. In dyeing with basic dyes, therefore, it is 
advisable to add sufficient acetic acid to the water before use to 
exactly neutralise the carbonates present. Of course this treat- 
ment is quite unnecessary when acid dyes are employed, as acid 
is usually added with the dye, and with dyewoods the presence 
of a little calcium salt is advantageous. 

As each " degree " of total hardness represents a soap- 
destroying power of at least 2 oz. of soap per 100 gallons of water, 
allowance must be made in making up " fat -liquors " with soap 
and oil for the loss of soap due to its precipitation by the mineral 
matter in the water. The sticky lime-soaps are apt to adhere 
to the leather and interfere with glazing, so that it is much 
better to employ a soft water. 

Permanent hardness of water is generally caused by sulphates 
of lime and magnesia, and more rarely by chlorides and nitrates. 
As none of these can be precipitated by lime, permanent hardness 
cannot be removed by Clark's process, nor can it produce the 
injurious effect on limed hides which have been attributed to 
temporary hardness. Neither can the lime and magnesia present 
combine with the tannins if used for leaching, since they are 
already fixed by stronger acids, and at most can only act in- 
juriously by slightly lessening the solubility of the tannins. 
Even this effect cannot be regarded as proved, though it deserves 
further investigation. ^ Permanent hardness is therefore of little 

1 The investigations of Nihoul (" Influence de la nature de I'eau sur 
r extraction des matiferes tannantes," Bull, de la Bourse aux cuirs de Liige, 
September 1901) on the tanning waters of Belgium seemed to show that 
permanent hardness is more inj urious in the extraction of tanning materials 
than has generally been supposed. If the tannins existed as sodium or 
potassium salts in liquors very free from acidity, it is quite conceivable 



WATER AS USED IN THE TANNERY 77 

moment as regards the ordinary uses of the tannery, though it 
has considerable influence in some of the processes of dyeing, and 
acts very injuriously where soap is used for scouring, as in the 
washing of sheep-skins for wool mats, since each part of lime 
reckoned as carbonate destroys at least twelve parts of pure 
soap (sodium stearate or oleate), producing a sticky and insoluble 
lime-soap which adheres to the fibre. In sole leather tanning 
permanent hardness is sometimes advantageous, especially if 
it be due to calcium and magnesium sulphates, and Vignon 
recommended that sulphuric acid should be added to the water 
before use in quantity sufficient to exactly neutralise the car- 
bonates which cause temporary hardness, as magnesium and 
calcium sulphates are not injurious, but tend to plump the 
hides. It must be remembered, however, that the carbonic acid 
liberated may still have prejudicial effects on limed hides. 

Permanent hardness is most objectionable in waters employed 
for boiler-feeding, and calcium sulphate is especially so, as 
it becomes nearly insoluble in water at 150° C. or 55 lb. 
steam-pressure, and is deposited on the plates as a hard crystal- 
line scale which must be chipped off with a hammer. Where 
many boilers have to be worked with a hard water, it is much 
the most satisfactory to soften the water with caustic soda, or 
with lime and soda together, before it comes into the boiler, 
but in cases where the plant required would be too costly, boiler- 
compositions are sometimes used with good effect, though con- 
siderable caution is advisable, since some of them affect the 
plates injuriously. The active constituent of many boiler- 
compositions is soda-ash or sodium carbonate, which acts by 
double decomposition with the calcium sulphate, forming sodium 
sulphate, and precipitating calcium carbonate as a sediment 
which is easily washed out. Most tanning materials, and even 
spent tan liquors, will prevent or lessen incrustation if mixed 
with the feed water, but sometimes corrode the plates if used too 
freely. This danger is lessened if they are used in conjunction 
with soda. Heavy mineral oils, either introduced in small 
quantity with the feed water, or painted on the sides of the boiler 
when cleaned, are useful in preventing the formation of a coherent 
scale. 

The removal of permanent hardness from water is easily 

that the}'- might react with calcium sulphate, forming insoluble calcium 
tannates. Wilson and Kern (" The Non-tannin Enigma," J.A.L.C.A., 
191 8) have shown that the effect of electrolytes which do not combine 
with hide-powder is invariably to raise the non-tannins, and so diminish 
the apparent amount of tannin. 



78 PRINCIPLES OF LEATHER MANUFACTURE 

effected in most of the forms of apparatus employed for the 
softening of water by. lime by using a calculated quantity of 
sodium carbonate in addition. The reaction is represented in 
the case of calcium sulphate by the following equation : — 

CaS04 + NaaCOg = CaC03 + Na2S04. 

The conversion of magnesium sulphate into carbonate may be 
similarly effected, but as the latter is somewhat soluble, an 
additional equivalent of lime must be used to precipitate it as 
hydrate. Magnesium salts, from their solubility, do not cause 
scale on boilers (though the chloride is apt to produce corrosion), 
but they are equally destructive of soap with the calcium salts. 
Caustic soda will remove temporary hardness, and after becoming 
converted into carbonate will further react on any permanent 
hardness present ; and its use is therefore sometimes convenient 
in small softening plants, but it is not more effective, and con- 
siderably more costly, than a suitable mixture of lime and sodium 
carbonate. Even with these, Archbutt states that the cost of 
softening permanent hardness is about ten times as great as that 
of removing temporary hardness with lime only. ^ 

Beside the method just mentioned, various others are described 
for the softening of permanent -hard waters (see pp. 71-75). 

As regards the influence of other impurities, our knowledge 
is far from complete, but the following are the most important 
matters likely to be present : — 

Mud under any circumstances is objectionable. It frequently 
contains organic slime and organisms which encourage the 
putrefaction of hides placed in it to wash or soften. It also 
almost invariably contains iron as one of its constituents, and 
hence stains leather and gives dark-coloured liquors. It is not 
easily removed by filtration, as large filter-beds are expensive 
and difficult to keep in order, and much space is required to 
clear water by subsidence. Some mechanical filter which can 
be easily cleaned, and used under pressure, offers the best chance 
of success. The Pulsometer Company make one, consisting of 
sponge tightly packed below a perforated piston. To cleanse 
the filter a stream of water is passed the reverse way, and the 
piston raised and worked up and down, either by hand or power, 
so as to loosen and knead the sponge. Filter-presses, in which 
cloths, or in some cases sand, are used as the filtering medium, 
are also well adapted for the purpose. If a water be softened 

1 Proceedings of Inst, of Mech. Engineers, 1898, pp. 404-54, in which 
much valuable information on water-softening is given. 



WATER AS USED IN THE TANNERY 79 

by Clark's or other process the precipitated chalk carries down the 
mud with it, together with most of the organisms. 

Iron is always an objectionable impurity in the tannery, 
though it is less injurious to the quality than the appearance of 
the leather produced, and indeed German sole leather tanners 
frequently put old iron in the handlers to darken the colour of 
the leather, and apparently, if not really, to quicken the tannage. 
It must not be present in waters used for dyeing. Iron oxide is 
frequently present as a mud merely, and in this case can be 
removed by filtration. It is rarely in solution in any other form 
than that 6i acid carbonate, since sulphate or chloride could not 
exist in presence of bicarbonate of lime. In this form, iron is 
precipitated at once by boiling or on the addition of lime, like 
the temporary hardness due to other bases, in the form of ferric 
hydrate, and more slowly by oxidation on exposure to the air. 
The mud produced by softening waters which contain iron must 
be completely removed by filtration, or subsidence, before the 
water is used for leaching,, or the iron will redissolve in the acids 
of the liquors. Iron is not perceptibly injurious in the limes, 
but in the bates and wash-pits sometimes causes stains, which 
are scarcely visible till blackened by the tanning liquors. In 
presence of sulphur (from sulphide of sodium or the decomposi- 
tion of sulphates by the sulphur-bacteria nearly always present 
in bates and soaks) the stains become bluish or greenish-black, 
and a black deposit is frequently produced on the sides of the 
pit, in which the threads of sulphur-bacteria (Thiothrix) can 
often be recognised by the microscope. As ferric salts not only 
combine with the tannins, but are themselves tanning agents 
(see p. 275), they are rapidly absorbed by leather, and iron is 
always present in leather ash. (For detection and estimation see 
L.I.L.B., p. 40 ; L.C.P.B., p. 23.) 

Alumina, except as clay, is rarely present in waters, and 
probably harmless in any water likely to be used in tanning. 

Soda is sometimes present in considerable amount, as sulphate, 
chloride, or carbonate. The sulphate slightly increases plumping 
in limes. The chloride, if present in material quantities, as in 
tidal rivers, lessens it, and may cause thin and soft leather, 
and in large amounts will greatly impede the proper exhaustion 
of many tanning materials. Sodium carbonate is sometimes 
present in considerable quantities, as in some of the waters of the 
Leeds district. It may coexist with temporary hardness, and 
produces similar injurious effects, but waters in which it is present 
cannot have any real permanent hardness. It may be neutralised 
by the very cautious addition of an acid, or by admixture of a 



8o PRINCIPLES OF LEATHER MANUFACTURE 

permanent-hard water. It tends to increase plumping in the 
hmes, but neutrahses the free acids of the tan liquors which are 
necessary in sole leather tanning. 

Copper, lead, and other metallic bases are not likely to be 
present in any waters used for tanning in quantities sufficient to 
be injurious. 

Sulphuric acid rarely occurs free in water, and then only in 
such traces as would be harmless for tanning, though possibly 
injurious to steam boilers. As sulphates it is most common. 
Alkaline sulphates are not known to have any deleterious action. 
The sulphates of lime and magnesia are the principal cause of 
permanent hardness, q.v. Iron sulphate is sometimes found in 
colliery waters, from the oxidation of pyrites. 

Nitrates and nitrites, in water are usually the result of " pre- 
vious " sewage contamination, and are only important as an 
indication of the possible presence of the putrefactive ferments, 
and are of little moment in waters only used for manufacturing 
purposes, while they seem to be even useful in promoting the 
" working " of bran drenches, by supplying the nitrogen required 
by the ferment. 

Chlorine is seldom or never present in water in the free state, 
but only in the form of chlorides, most frequently that of 
sodium (common salt), the effect of which has been referred to 
on p. 79. The action of other chlorides is probably similar 
as regards the swelling of hide. Magnesium chloride is very 
objectionable as a constituent of boiler-waters, as it liberates 
hydrochloric acid at high temperatures, and corrodes the plates 
at the surface of the water. This injury can be prevented by 
addition of soda. 

Carbonic acid has been referred to under temporary hardness. 
Its presence in the free state is a matter of some importance to 
the tanner (see p. 191). 

Silicic acid in a soluble form is present in some waters in con- 
siderable quantity. Such waters are said to harden leather, but 
of this the writer has no personal experience. 

Few accurate researches have been made on the effect of the 
impurities of water on tanning, ^ and though, from what has 
already been said, it will be seen that they are not without effect, 
it is probable that in many cases the water is blamed for troubles 
which are simply the result of mismanagement, and credited with 
virtues which are really due to careful and skilful manufacture. 

^ See Nihoul, loc. cit. ; also Report of American Commission on " Effect 
of Hard Water on Tannins," J.A.L.C.A., 1918, p. 409; and Wilson and 
Kern, loc. cit. 



WATER AS USED IN THE TANNERY 8i 

The hardness of water, and the dissolved carbonic acid which 
it contains, are, together with its temperature, the principal 
factors which determine whether a hide will plump or fall in it. 
Almost the only careful investigation of this point has been 
made by W. Eitner.^ He placed pieces of hide, unhaired by 
sweating, and quite flat and fallen, in water for four days at a 
temperature of 46° F. (8° C.) with the following results : — 



1. In distilled water . . 

2. ,, water saturated with COg . 

3. ,, ,, with lime bicarbon- 

ate, 20° German 
scale of hardness 
magnesia bicar- 
bonate, 20° do. 

5. ,, ,, ,, lime sulphate, 

20° do. . 

6. ,, ,, ,, magnesia sul- 
phate, 20° do. 

magnesium chlor- 
ide, 20° do. . 

common salt, 
20° do. . 

(20° German scale equal 36° or parts 
see p. 67.) 



Scarcely at all plumped. 
Well plumped. 



Tolerably plump. 

Well plumped. 
Best plumped. 
Not at all plumped. 

of CaCOg per 100,000 



The peculiarities which were shown by the hide pieces on 
removal from the water were maintained throughout the tanning, 
which was conducted in imitation of the Austrian method, the 
hide being swollen and coloured through in weak birch-bark 
liquors, made with distilled water and acidified in each case with 
equal quantities of lactic acid, and finally laid away, till tanned, 
in a mixture of oak bark and valonia. No. 6, from magnesium 
sulphate, was the best ; then No. 2 ; No. 3 was less good, but 
all the pieces from i to 6 were firm, close, and of good substance 
and texture, No. i having swelled well in the sour liquor. On 
the other hand, 7 and 8 scarcely swelled in liquor, but remained 
fiat throughout, and were looser, thinner, and of finer fibre. From 
this experiment it is clear that while sulphates and carbonates 
exert a^favourable influence on plumping, chlorides do the reverse, 
as they themselves not only do not plump, but they place the 
hides in an unfavourable condition for the plumping action of 
acids in the liquors. These experiments are quite borne out by 
the writer's experience in practice. The water at the Lowlights 



^ Gerber (1877), iii. p. 183 ; cp. also Chap. X. 



82 PRINCIPLES OF LEATHER MANUFACTURE 

Tannery, which in dry weather was mostly obtained from beds 
of what was originally sea-sand, and which consequently contained 
a very abnormal proportion of chlorides (up to 68 parts NaCl per 
100,000), required special and very careful management to make 
thick leather, notwithstanding the fact that it contained a con- 
siderable quantity of calcium and magnesium sulphates. These 
facts also indicate the importance of the thorough removal of salt 
from hides intended for sole leather. Plumping is not a desirable 
thing in leather intended for dressing purposes, and it is possible 
that the use of a small percentage of salt in the liquors or wash 
waters might in some cases enable bating to be dispensed with. 

There is no practicable means of removing chlorides from water, 
but Eitner suggests the addition of a small quantity of sulphuric 
acid to water containing much temporary hardness (bicarbonates), 
•in order to convert it into permanent hardness (sulphates), which, 
as stated above, plumps better. The amount required may be 
calculated from an acidimetric determination of temporary hard- 
ness (see L.I.L.B., p. 19 ; L.C.P.B., p. 16). A simple, but not very 
accurate, guide is to add enough acid to purple, but not to redden 
litmus paper even after moving the latter about in the water for 
some minutes. In practice the acid must of course be very 
thoroughly mixed with the water by stirring and plunging. It must 
be borne in mind that Eitner's experiment was on sweated hides, 
and that with limed hide, which is kept plump by the dissolved 
lime retained in the hide, different results as regards carbonic acid 
and bicarbonates would be obtained. Both these would convert 
the lime in the hide into chalk, which is insoluble and inert, and 
the hide would fall, at any rate when the lime was completely 
carbonated, while hides would remain plumpest in waters m_ost 
free from substances capable of neutralising lime. From this 
we might conclude, what may be a priori expected, that the purer 
the water, the plumper limed hides remain in it, but in soft but 
peaty waters hides fall rapidly, from the neutralisation of the 
lime by the weak organic acids of the peat. Such waters are 
dangerous for domestic use from their solvent action on lead, 
but this danger can be entirely removed by storing the water in 
limestone reservoirs, or allowing it to flow slowly through a 
limestone culvert before use. In some towns in the north of 
England a small quantity of lime is added so as to neutralise the 
water as it leaves the reservoir and before it enters the mains. 

Wherever the conditions of putrefaction or decajdng organic 
matter are present, as in a bate, hides fall rapidly, and in extreme 
cases even the presence of the stronger acids wiU not maintain 
plumpness. Eitner mentions the case of a stream at Vissoko in 



WATER AS USED IN THE TANNERY 83 

Bosnia, which was in special repute among the tanners from its 
power of pulHng down hides rapidly, and which took its rise in 
a common on which the pigs of the town were pastured. The 
causes of this action are no doubt due to the products of putre- 
faction, but are somewhat obscure. Bacteria present in water 
are a frequent source of injury in the soaks, and probably in other 
stages of the tanning process. 

Rain water and the water of streams in mountain districts of 
hard igneous rock are generally nearly free from mineral con- 
stituents. This is the case with the Glasgow water from Loch 
Katrine, and the Thirlmere water which supplies Manchester. 
Such water, if cold enough, and free from mud and organic im- 
purity, is the best for almost every purpose in the tannery. Most 
river-water contains material quantities of mineral matter, though 
it is usually softer than that of springs or wells. 

For further details as to the chemical examination of water, 
and the methods of determining the amounts of its different con- 
stituents, see L.I.L.B., pp. 18 et seq., and L.C.P.B., p. 16. 



CHAPTER IX 

PHYSICAL CHEMISTRY IN APPLICATION TO LEATHER 
MANUFACTURE 

Though all sciences are really parts of one great whole, its 
immense extent makes it necessary to specialise, and to separate 
by more or less arbitrary divisions. One of the latest of these 
is Physical Chemistry, and it is rapidly increasing in both 
scientific and technical importance, though its limits are not easy 
to define. While the general chemistry of the past dealt mainly 
with the properties and reactions of elem-cnts and their com- 
pounds, and was satisfied to state that under given conditions 
such and such actions would ta.ke place, physical chemistry seeks 
to find the underlying laws of these changes, and to explain 
them by mechanical and physical causes which can be exactly 
and mathematically expressed. In general chemistry the atomic 
theory, the theory of valency, and the laws of gas and vapour 
pressures represent the beginnings of physical chemistry, but it 
now embraces large portions of the kindred sciences of electricity 
and thermodynamics, and some knowledge of physics is necessary 
before they can be understood. 

As it cannot be assumed that all readers of this work are 
familiar with these ideas, it is necessary to treat them in an 
elementary and explanatory way before attempting to apply 
them to the problems of leather manufacture, a large part of 
which depend on physical chemistry for their elucidation. Mathe- 
matics is as far as possible avoided, and where used is always of 
the most elementary sort. 

Constitution of Matter. — It is one of the fundamental ideas of 
general chemistry that all compounds are built up of atoms, 
but modern chemistry has taken a further step, and explains that 
the atoms themselves are built up of electrons, which are con- 
ceived as particles of electricity itself. These electrons are both 
negative and positive, and it is usually supposed that one or more 
large positive electrons occupy the centre of the system, with a 
number of negative electrons grouped round them. This view has 
been greatly strengthened by the fact that elements like radium 
give off a constant stream of electrons, and in doing so undergo 
change themselves, radium itself becoming ultimately converted 



PHYSICAL CHEMISTHY IN TANNING 85 

into a form of lead. The electrons themselves are minute charges 
of + or — electricity, and of course exert the usual attractions 
on those of opposite sign, which constitute chemical affinity. 
Electric currents are, in all probability, streams of free elec- 
trons. 

States of Matter. — Accepting the ordinary chemical theory, we 
assume that all matter as we know it is built up of molecules 
consisting of groups of atoms held together by their chemical 
(electrical) affinities ; and that for any individual substance these 
molecules are all exactly alike, containing the same number of 
the same atoms, arranged in the same way. We may now try to 
form some mechanical conception of matter as we see and feel it 
in its various forms — solid, liquid, and gaseous. In no case are 
the molecules at rest, but always in a state of rapid vibratory 
motion which we call heat, and the higher the temperature the 
more rapid is the movement of the molecules, and the greater 
the spaces through which they move. They strike each other, 
and rebound, and tend to separate, but, on the other hand, they 
are drawn together by an attraction very similar to, if not identical 
with, the attraction of gravitation, and very possibly electrical in 
its origin. This attraction is very powerful when the molecules 
are closely approached, but, like gravitation, rapidly diminishes 
with distance. These attractions are opposed by the energy of 
heat, which thus takes the same part in molecular physics which 
the energy of planetary motion does in the solar system. 

Gases. — In the gaseous or vapour state the spaces between the 
particles are so great that their attractions are inconsiderable, 
and if unconfined they would fly off into space, while their blows 
on the walls of any vessel which contains them are the cause of 
their pressure. The pressure is thus proportionate to the number 
of blows per second, and consequently to the number of gas- 
molecules in a given space, and if we double the density of gas, 
we therefore double the pressure (Boyle's law). Temperature is, 
in fact, most accurately measured by the pressure (or its re- 
ciprocal, the volume) of a given quantity of gas, each degree 
Centigrade corresponding to an increase of volume or pressure 
1/273 of that at 0° C. or freezing point (Gay Lussac's or Dalton's 
law). From this law a remarkable consequence foUows : at 
— 273° C, if the law holds, a gas would have neither pressure nor 
volume, or, in other words, the molecules would have none of the 
motion we know as heat, and hence —273° C. is known as the 
" absolute zero of temperature." Temperatures measured from 
this point are generally denoted by T, and it is found that 
measured in this way many relations are much simplified. It 



86 PRINCIPLES OF LEATHER MANUFACTURE 

is of course impossible to reach absolute zero, but in liquid 
hydrogen it is approached within a few degrees. ^ 

If different gases are confined together in the same space they 
will of course take the same temperature, and the molecules 
of each will exert their pressure on the walls independently of 
the others, so that the total pressure will be the sum of the 
" partial " pressures of each of the gases. It can be shown also 
that each molecule must on the average acquire the same energy 
of motion, whatever its weight, so that it follows that the 
heavy molecules must move more slowly than the lighter, and 
that all molecules, whatever their weight, must exert the same 
pressure, or at constant pressure must occupy the same volume. 
This is Avogadro's law, and the fact that at a given temperature 
and pressure there must be the same number of molecules of any 
gas or vapour in the same space is constantly used to determine 
the relative " molecular weight " of different substances. 

The Liquid State differs from the gaseous in that, though 
mobile, it can be retained in an open vessel, and can only escape 
by taking the form of vapour or gas. We may conceive that the 
molecules of liquids, though still possessing heat-motion, are so 
much closer together that their attraction is sufficient to prevent 
their separation, though they can still move within a limited 
distance. Their motion can indeed be shown indirectly by the 
constant vibration of small particles, such as bacteria (Brownian 
motion), which can be seen in the microscope, and which is caused 
by the bombardment of the liquid molecules. 

Vapour-pressure. — Though the average velocity of the molecules 
is a constant at any given temperature, individual molecules 
may have their velocity increased or diminished by collision with 
others, so that a certain proportion, if they reach the surface, 
may pass through it and take the form of vapour or gas. On 

1 These laws of perfect gases are mathematically expressed by the 
equation pv = RT, where p is the pressure, v the volume, T the absolute 
temperature, and R the "gas-constant," which is the pxv of a gram- 
molecule of gas at I ° T and unit pressure, and of course varies with the units 
used. If pressure is in grams per cm.^, and volume measured in c.c, 
R=84,736. This equation is only approximately true of actual gases, 
though very nearly so of hydrogen, and the deviation is only serious as 
the point of liquefaction is approached, firstly, because the molecules them- 
selves have an actual volume, and, secondly, because they approach each 
other so closely that their attractions can no longer be neglected. The 
equation of van der Waals introduces corrections for these, and is approxi- 
mately true even of the liquid state. It is lp+ -]{v—b)=KT, a being 
a constant of attraction, and b the actual volume of the molecules. 



PHYSICAL CHEMISTRY IN TANNING 87 

the other hand, vapour-molecules, striking the surface, may 
be entrapped by its attraction and revert to the liquid form. 
Of course the denser the vapour the larger the number of 
molecules will be entrapped in this way, while the hotter the 
liquid the more will escape, and thus a perfectly definite " vapour- 
pressure " will be established for a temperature at which these 
numbers are equal, in spite of the fact that it is merely an average, 
and that individual particles are constantly passing from one 
state to the other. Such " equilibria " are common enough in 
daily life, as, for instance, in the comparative steadiness of the 
birth and death rates in spite of all sorts of interfering causes, and 
business depends to a large extent on the steadiness of averages. 
We shall meet with many similar equilibria in the course of our 
chemical study. 

The vapour-pressure depends on temperature only, and is 
not affected by the presence of other gases or vapours in the space, 
each gas or vapour having its own partial pressure. If water be 
introduced into an exhausted flask, the space will become filled 
with water-vapour at a pressure solely depending on its tempera- 
ture, and at boiling temperature equal to that of the air outside. 
If the flask were filled with air at atmospheric pressure before the 
water was introduced and then sealed, the pressure would be 
that of the air in addition to that of the water-vapour, and at 
boiling point might be double that of the outside air, and 
explosions have been attributed to that cause. 

Critical State. — If the liquid and its vapour be gradually heated 
in a closed vessel, the density of the vapour wiU increase with 
its pressure, while that of the liquid will diminish with its heat- 
expansion, so that a point will be reached when the two are 
identical, the so-called " critical temperature," above which there 
is no distinction or separation between liquid and vapour, and 
no " liquid surface." Above this temperature, liquefaction is 
therefore impossible, and it was want of knowledge of this which 
caused failures of all early attempts to liquefy the more " perfect " 
gases, of which the critical temperature is very low, that of 
hydrogen being —234-5° C. That of water is +365° C, and its 
pressure is 200 atmospheres, or about 2940 lb. per square inch. 

Boiling Point. — The boiling point of a liquid is reached when 
its vapour-pressure exceeds that of the air above it and is able 
to form bubbles in the interior, and it is therefore dependent on 
the air-pressure. Thus in a vacuum pan water may boil freely 
at very low temperatures. 

Heat of Evaporation. — When a liquid in an open pan over a 
gas-burner reaches a boil, no length of time will raise its tem- 



88 PRINCIPLES OF LEATHER MANUFACTURE. 

perature any higher, the whole heat of the burner being expended 
on the work of evaporation, that is, in overcoming the internal 
attractions of the liquid, and setting the molecules free as vapour. 
The quantity of heat consumed in this way is very large ; loo 
calories (p. 514) are required to raise a kilogramme of water from 
0° C. to 100°, but 536 calories to convert it into steam at the same 
temperature. (A large amount of heat (80 calories) is also 
consumed in the melting of ice at freezing point.) 

Internal Pressure. — From the attractions which have just been 
mentioned, the interior of a liquid is subject to very heavy 
pressures, but these are only exerted on things actually forming 
part of the liquid, that is, really dissolved in it, and not on what 
is merely dipped into it, which from this point of view is still 
outside it. For this reason internal pressures cannot be directly 
measured, but they can be calculated with some accuracy from 
the heat of evaporation, since Stefan has shown that to bring 
a particle actually into the surface consumes half the work which 
is required to set it entirely free in the form of vapour. For 
boiling ether the internal pressure is 1284 atmospheres, and for 
water at freezing point 11,000 atmospheres, or about 72 tons per 
square inch. Though the internal pressure cannot be directly 
measured, its effects are visible in several ways, as, for instance, 
in the large force required to tear liquids apart when free from 
gas-particles, and in the violent "bumping" of boiling gas-free 
liquids, of which the pressure must rise much above that of the 
atmosphere before it can form a fresh bubble. 

Surface-tension is also a consequence of these forces. Although 
they are of such great intensity, the attraction rapidly diminishes 
with distance, and the surface-layer in which they have been 
partially overcome is only of a thickness to be measured in 
millionths of a millimeter, while throughout the rest of the liquid 
they are undiminished. If, therefore, we imagine Stefan's 
particle actually half out of the surface, it remains attracted by 
only those particles beside and below it within this very short 
range, while above it there are only vapour particles beyond its 
sphere of attraction. By the ordinary mathematical device of 
the parallelogram of forces we can represent or " resolve " all 
these attractions into a vertical component pulling directly 
downwards, and horizontal components of much smaller amount 
pulling the surface together equally in all directions. These 
small horizontal components constitute the force known as 
" surface-tension." In water, in which it is larger than in almost 
any other liquid, it only amounts to about 75 milligrams across 
a surface i centimeter wide. 



PHYSICAL CHEMISTRY IN TANNING 89 

Though the force is thus small, its effects are visible in many 
ways. Certain insects walk on the surface of water as though 
it were covered by an elastic rubber sheet, and a drop or a bubble 
will contract and draw back if the pressure within it is lessened. 
Perhaps its most important effect is that of capillarity. A liquid 
will rise in a wet tube of small diameter considerably above the 
level of the surface outside, and in the same way oil is sucked up 
by a wick, or absorbed by leather or fabric. In a wider tube 
the liquid is drawn up round the walls, so that the surface is 
not flat, but takes the well-known meniscus shape. In a vessel 
which the liquid does not wet, on the other hand, the edges of the 
liquid are depressed, and the surface is convex, like that of the 
mercury in a barometer. 

Surface-tension may be measured by the extent to which the 
given liquid will rise in a small tube, but for most purposes the 
easiest method is that of weighing a given number, say, 100 drops, 
which are allowed to fall slowly from a small horizontal opening, 
such as the bottom of a burette. As it is not easy to measure 
exactly the outside diameter of the dropping tube, it is simplest 
to make a preliminary experiment with water, of which the 
surface-tension may be assumed as 75, and calculate that of 
other liquids measured with the same tube as proportional to 
the weight of drops as compared with those of the water. The 
dropping end of the tube should be thin, smooth, and quite free 
from grease. Very small quantities of some impurities, especially 
organic, greatly affect the tension. 

Much controversy has arisen as to whether the actual surface 
is a region of increased or diminished pressure, and elaborate 
theories of tanning, and of colloids generally, have been based 
on the former assumption, which, in the writer's opinion, is 
entirely unfounded and unsupported by facts. It need only be 
said that if the explanation of the causes of surface-tension which 
have been given here, and which are held by the most eminent 
authorities on the subject, and especially by van der Waals and 
his pupils, are correct, the surface is a region not of increased but 
of diminished pressure and density, and that the actual boundary 
between the liquid and its vapour is a sort of critical layer where 
the liquid escaping from the internal pressure is of an equal 
density with the vapour condensed by the attraction of the liquid 
surface. 1 

Effect of Dissolved Substances on Surface-tension.— CertSLin sub- 
stances, especially salts, increase both the surface-tension and the 

^ Cp. Procter, " The Nature of the Liquid Surface," J.S.L.T.C., 3, 1919, 
p. 48. 



90 PRINCIPLES OF LEATHER MANUFACTURE 

internal pressure. Others, of small surface-tension themselves, 
naturally lower that of water when dissolved in it, and some 
organic matters, such as soaps, saponins, etc., reduce it to an 
extraordinary degree. 

Surface-energy is equal to the area of surface x the surface- 
tension, and it is an important law that it tends to reduce 
itself to a minimum in every possible way, either by diminish- 
ing the surface or reducing the surface-tension. Thus a drop 
of water falling freely, or of melted lead in a shot-tower, 
always takes the spherical form, since this possesses the 
smallest surface in proportion to its volume ; and for the same 
reason, globules of one liquid suspended in another are always 
spherical. The same law also demands that if two globules of 
liquid come in actual contact they must coalesce, since the 
surface of the larger volume is less than that of the two smaller 
one.^. Another consequence is the law of Willard Gibbs, that if 
a solution contains a constituent which lowers surface-tension 
this must accumulate in the surface, and this has been shown 
experimentally to be the case, and is one of the causes of adsorp- 
tion, under which heading it must be further noticed. The same 
fact introduces a difficulty into the measurement of the surface- 
tension of impure liquids, since the tension diminishes if the 
surface stands long enough for the impurity to accumulate in 
it, and lower results are obtained by the ordinary " statical " 
methods than by " dynamic " methods, in which the surface 
is constantly renewed. Some organic matters, especially the 
saponins, not merely accumulate in the surface, but appear to 
become coagulated there and form a coherent film. 

So far we have considered only liquid surfaces in contact with 
their own vapour, or with inert gases the influence of which may 
generally be disregarded, but classes of surfaces (or, more strictly, 
interfaces) very important for technical purposes are those in 
which one liquid is in contact with another immiscible with it, 
or with a solid. Petrol or ether upon water, or water upon 
chloroform, are instances of the first case, while any solid immersed 
in a liquid presents the second. If a little petrol be poured on 
water in a test-tube it will be noted that the meniscus between 
them still exists, but is flatter than that on the surface of either 
in contact with air. The reason is that a portion of the attraction 
of one liquid is exerted on the other and thus partially com- 
pensated, and the surface-tension of both is lessened, that of 
the lesser becoming actually minus, and the tension of the 
interface is merely the difference between them. This rule is 
borne out by experiment if it is remembered that practically all 



PHYSICAL CHEMISTRY IN TANNING 91 

liquids have some mutual solubility, and that the liquids actually 
in contact are therefore the saturated solutions of each in the 
other, and not the pure liquids themselves. 

It has been mentioned that when a liquid of higher surface- 
tension is in contact with one of lower, the surface-tension of the 
latter becomes a negative quantity — that is, that the surface 
tends to spread instead of to contract. It thus follows that one 
liquid should always spread on another, and it is necessary to 
explain some apparent exceptions. When an oil in any quantity 
is poured on water, it often forms drops instead of spreading 
further. The cause is that the whole surface of the v/ater very 
rapidly becomes coated with an extremely thin and invisible 
film of oil, on which the remaining oil has no further tendency 
to spread. The same is true of soHd surfaces, which have pre- 
sumably a very high though non-measurable tension. It has 
been shown that water will always spread on a really clean 
solid surface, but the least trace of grease will prevent it. 

There is still the further, and probably very important, case of 
three liquids of different tensions, or two liquids and a solid. In 
this case the liquid of lowest tension, tending to spread on the 
surfaces of both the others, will spread between them, or at least 
attach itself to both surfaces when exposed, and so form an 
intermediate film. It is very probable that this explains the use 
of water in stuffing leather, and is not unimportant in the forma- 
tion of emulsions. 

Emulsions are intimate mixtures of two liquids which are not 
mutually soluble, such as oil and water. If oil be shaken with 
water it is broken into small globules, which remain suspended 
for a time, but ultimately float up to the surface, and again 
coalesce to a liquid layer. If the globules are extremely small 
this takes place very slowly, owing to the small effect of gravity 
and the friction which the particles experience in passing through 
the water ; and possibly also because they may take similar 
electric charges which cause mutual repulsion. The addition of 
viscous substances, such as gum or starch, renders such emulsions 
more permanent by increasing the liquid friction ; and even 
finely divided powders may have a similar effect by coating the 
globules and preventing their coalescence. (See p. 472.) 

The less the difference in the surface-tensions of the two liquids 
the more readily such emulsions are formed, and if therefore a 
third substance, such as soap or saponin, which lowers surface- 
tension and often forms surface-films, is added, it greatly 
facilitates the operation and conduces to the permanence of the 
emulsion. Small quantities of alkali have a similar effect on oils. 



92 PRINCIPLES OF LEATHER MANUFACTURE 

saponifying the traces of free fatty acids which are always present. 
It is highly probable that these third substances act also by coating 
the globules when formed, as has been explained in the preceding 
paragraph, and so preventing their coalescence, and rendering 
the emulsion permanent. The bearing of these facts in the 
preparation of fat-liquors is obvious. Some oils are much more 
readily emulsifiable than others, especially if they contain free 
fatty acids or some oxidised fats, as is the case with sod oil and 
degras. Addition of a trace of oleic acid to a neutral oil is often 
useful. Sulphonated oils are also very powerful emulsifying 
agents. The author has examined a sample of mineral oil con- 
taining at most 20 per cent, of sulphonated oil which emulsified 
spontaneously to a milky liquid when poured into water. Sul- 
phonated and oxidised oils all contain traces of water-soluble 
substances which probably play the part of the intermediate 
coating substance. 

It must not be forgotten that in any emulsion of two liquids 
either may form the separate globules or " disperse phase." 

Solution.- — Emulsions or suspensions of solids may be so finely 
divided as to possess most of the properties of solutions, and will 
be considered later under the head of Colloids, but true solutions 
may be regarded as mixtures so intimate that the individual 
molecules of the dissolved body come within the sphere of 
attraction of those of the solvent, and take part in its internal 
pressure, thus forming a single homogeneous liquid. 

Many liquids will mix or dissolve in each other in any propor- 
tions, e.g. water and alcohol ; the attraction of the alcohol for 
the water-molecule being as great or greater than that of alcohol 
for alcohol, or water for water. In other cases, such as water and 
oil, or water and petroleum spirit, practically no mixture takes 
place, their mutual attraction being small ; and each retains a 
considerable surface-tension at the points of contact, though 
less than that of the free surfaces, since each exerts an attraction 
on the other. There are also many intermediate cases, such as 
water with chloroform, carbolic acid, or ether, in which each 
solvent dissolves a portion of the other, but the two solutions 
do not mix, but form separate layers. In these cases an equili- 
brium is attained, in which there is just as much tendency for 
either of the liquids to pass into as out of the other layer. In this 
there is an extraordinary resemblance to what has been said of 
vapour-pressures ; and the tendency to pass into solution is often 
called solution-pressure ; and it may be noted that when equili- 
brium has been reached, not only is the solution-pressure but 
the vapour-pressure of each constituent equal in both solutions. 



PHYSICAL CHEMISTRY IN TANNING 93 

Like vapour-pressures, the solution-pressures usually increase 
with rise of temperature, more of each constituent passing into 
the other, till at last the composition of the two layers becomes 
identical, their surface-tensions disappear, and complete mixture 
takes place. With phenol (carbolic acid) and water this takes 
place at about 70° C. The similarity of this to the miscibility 
of liquid and vapour at the critical temperature is very striking, 
and the point where complete mixture occurs has been called 
the critical temperature of the two solutions. In these liquid 
mixtures either body may be considered as the solvent, or the 
dissolved substance. The distinction is quite arbitrary, but 
usually that present in largest quantity is called the solvent. 

Distribution between two Solvents. — -If a substance is soluble in 
both of two immiscible solvents, it will be distributed between 
them in a constant ratio, according to its solubility in each, what- 
ever its quantity, and this ratio is generally called the " partition 
constant." The point is important in the separation of sub- 
stances by " shaking out," as, for instance, the extraction of 
tannin from aqueous solution b}^ acetic ether, as it enables 
us to predict what quantity will be removed by successive 
shakings. 

If, as sometimes happens, the " molecular complexity " is 
different in the two liquids, as in the case of acetic acid, where 
the molecules are twice as large in benzene as in water, the 
ratio of concentration, instead of being simply C^/Cj,, as in the 



1 



I 



first case, will be C„/C,,«, in which the fractional power — will be 

n 

that of the relation of the two molecular weights ; thus for the 
case mentioned it will be C^/Cb^ or C^^/Cb, where w is the water 
and B the benzene solution. This case is perhaps not of much 
consequence to the tannery chemist, but is mentioned because 
of its analogy to what is called the " adsorption formula." ^ 

Adsorption. — If finely divided charcoal, or many other fine 
powders, be shaken up with dilute solutions, they will in many 
cases, dependent on the nature of the dissolved substance, fix a 
portion on their surface much larger than its concentration in 
the liquid, and larger in proportion in dilute than in more con- 
centrated solutions. Thus bone-charcoal is constantly used to 
decolorise sugar solutions, and charcoal will also remove con- 
siderable quantities of tannin from aqueous solution. The pro- 
portion removed can generally be stated by a formula of the 

1 The divisor in a fractional power is a "root" ; thus C\ is the square 
root of C, or -^/c. Cf is the cube-root of C squared, and so on. 



94 PRINCIPLES OF LEATHER MANUFACTURE 

X I 

form — ==aC«/ where x is the weight of substance absorbed, m 
m 

that of the absorbent, a a constant varying with the substance, 

C the concentration of the solution, and — a fractional index, 

n. 

often about |, also varying with the dissolved substance. 

It has been shown by Willard Gibbs, that any substance which 
lowers the surface-tension must necessarily accumulate in the 
surface ; and experiments with froth, and other liquid-air or 
liquid-vapour surfaces, have confirmed his theoretical conclusion. 
It has been constantly assumed by colloid chemists that the same 
reasoning must apply to the contact of liquids with solids, but 
this is by no means obvious, since in such cases the liquid surface 
has a negative tension, and tends to spread on the solid. For 
a full discussion of the theory of adsorption, see Freundlich's 
" Kapillarchemie " and other books on colloid chemistry. 

Most of what has been said about liquid solutions is also true 
of solutions of those solids known as " crystalloids." 

Sohdion of Solids. — Matter exists in two states known as 
" crystalloid " and " colloid." The colloid state is amorphous, 
and in solution has no definite point of saturation, and in most 
(probably in all) cases its particles are larger than single molecules, 
and are often known to be aggregates ; and its solutions are 
emulsions or suspensions rather than true solutions. It is, 
however, of such importance in leather manufacture, that it will 
be dealt with in a separate and later chapter, since most of the 
materials of leather manufacture exist in the colloid state. 

^ This formula has received no theoretical explanation, and probably 
has none. It is applicable to a great variety of chemical reactions, which 
proceed rapidly at first, and gradually decrease. It is frequently called 
the " adsorption-isotherm," which itself is a misnomer, as an isotherm is 
a curve of equal temperature, yet many people seem to think that because 
the course of a reaction can be approximately represented by the formula 
it is a proof that the reaction is physical and not chemical, and may be 
classed as a surface-phenomenon, a conclusion by no means justified, 
since it will approximately represent many which are obviously chemical. 
It is a mathematical consequence of the form of the formula, that if, 

X 1 

instead of plotting — and aC^ we plot their logarithms, the result will be 

m 

a straight line inclined to the axis at the angle represented by - . This is 

rarely the case in actual adsorption experiments, the line being almost 
invariably more or less curved. (It must not be forgotten in plotting 
negative logarithms, that in the tables, only the index is negative, while 
the mantissa remains -|-, and to get the true negative logarithm must be 
subtracted from the index.) 



PHYSICAL CHEMISTRY IN TANNING 95 

Solids in the crystalloid state are characterised by regular 
crystaUine form, indicating that the attractive forces of their 
molecules are exerted in definite directions, giving them a ten- 
dency to attach themselves together in regular geometrical 
arrangements. They themselves dissolve no part of the solvent 
(though sometimes a definite number of molecules of the solvent 
combine in forming the crystal, as " water of crystallisation "), 
but are dissolved by it till an equilibrium is reached in which the 
tendency of further particles of the solid to pass into the solvent 
is balanced by that of those already dissolved to attach them- 
selves to the remaining solid, or " crystalhse out." Such a 
solution is " saturated " with respect to the sohd residue, but the 
word has no meaning unless some second immiscible phase, such 
as solid crystals, is present, and where a body has, as sometimes 
happens, more than one crystalline form, a solution may be 
saturated with regard to one of them, and more or less than 
saturated with regard to another. In " supersaturated " solu- 
tions, crystallisation is at once started by the addition of a " seed " 
crystal of the proper form. This is well seen by dissolving 
sodium sulphate or thiosulphate in boiling water, and allowing the 
solution to cool in a flask plugged with cotton-wool to exclude 
dust. If the minutest crystal of the solid salt is now dropped in, 
the whole immediately solidifies with evolution of heat. 

If a crystalloid substance, such, for instance, as copper sul- 
phate, be placed in a solvent {e.g. water), the dissolved salt will 
gradually spread itself through the whole body of the solvent, 
though in the complete absence of currents in the liquid the 
motion is extremely slow, and years may be taken for the diffusion 
to rise through a few feet. In many cases salts diffuse through 
aqueous jellies at nearly the same speed as they would through 
still water. Colloid substances, on the other hand, have httle or 
no power of diffusion, and mostly cannot pass through jellies at 
all. This is the reason why tannage with mineral salts is so much 
more rapid than with vegetable tannins, which are of colloid 
character, and which diffuse through the gelatinous fibres of the 
hide with extreme slowness. 

Osmotic Pressure. — All dissolved crystalloids do not pass 
through gelatinous membranes with equal ease, and substances 
are known, mostly gelatinous precipitates, which do not permit 
the diffusion of many dissolved salts, though they allow water 
to pass freely. Thin layers of such precipitates form what are 
called " semipermeable membranes." The existence of such 
membranes affords us the possibiHty of direct measurement of 
the tendency to diffusion, or, as it is generally called, the 



96 PRINCIPLES OF LEATHER MANUFACTURE 

" osmotic " 1 pressure of dissolved bodies. Thus a porous 
earthenware battery-cell may be immersed in a solution of copper 
sulphate, and filled with one of potassium ferrocyanide. In this 
way its pores will be filled with a gelatinous precipitate of copper 
ferrocyanide, which is pervious to water, but impervious to most 
dissolved substances. If now the cell be filled with a dilute 
solution of some crystalloid, say sugar, and its top closed by a 
perforated cork fitted with a vertical tube, and the cell be placed 
in water, the latter will pass into the cell, and the dilute solution 
will rise in the tube to a height, of many feet above the water 
outside. By substituting a mercury pressure-gauge for the 
vertical tube exact measures of the pressure in the cell can be 
made, which is the osmotic pressure of the dissolved substance. 
At first sight it is paradoxical that water should flow into the 
solution, apparently against a heavy pressure, but the explana- 
tion is simple. Mention has already been made of the enormous 
internal pressures of liquids produced by the attractions of their 
molecules. In the solution a portion of this is borne by the 
osmotic pressure of the dissolved substance, and the water flows 
in from the outside till an internal mechanical pressure is pro- 
duced equal in amount to this osmotic pressure. The resem- 
blance of the phenomena of solution to those of vapour-pressure 
is very obvious, and it is found to be even quantitative, since 
the measured osmotic pressures are exactly equal in amount to 
those which the dissolved body would produce if it were in the 
state of vapour at the same temperature and occupying the same 
volume as the solution. It acts, in fact, precisely as the " partial 
pressure " of a vapour. There are several indirect ways of 
measuring the osmotic pressure of dissolved bodies, as, for in- 
stance, from the lowering of the freezing point, or the raising of 
the boiling point of the solution as compared to those of the pure 
solvent, which are easier of execution and generally more exact 
than the direct measurements, and show that in a given volume 
at the same temperature the same number of molecules will produce 
the same osmotic pressure whatever their nature, or conversely, 
that at the same osmotic pressure and temperature equal volumes 
of any solution must contain the same number of molecules. The 
use of these facts in determining molecular weight is obvious. 

1 Solution-pressure and osmotic pressure are really two names for the 
same force when the solid is present in the solution ; the former being 
employed to signify the tendency of a solid to dissolve, and the latter the 
pressure produced by the dissolved body which tends to prevent further 
solution. Thus, in a saturated solution in contact with its solid, the two 
pressures are always equal, but exerted in opposite directions. 



PHYSICAL CHEMISTRY IN TANNING 97 

Electrolysis. — Many liquids do not conduct electricity, and 
pure water only does so to a very limited extent, but many 
solutions conduct freely. The substances which do so in solution 
are called " electrolytes." The conduction is of quite a different 
character to that in a metallic wire, which undergoes no per- 
manent change in the process, while that of the electrolyte is 
always accompanied by chemical decomposition, and the actual 
transference of matter from one pole or "electrode" to the 
other. If, for instance, the solution is one of silver nitrate, a 
certain quantity of electricity will be required to deposit a gram- 
equivalent of silver on the kathode or, as it is generally termed 
the " negative " pole, while an equivalent of nitric acid (or more 
strictly of NO3) will be liberated at the anode or " positive " 
pole. If the same current passes through a solution of copper 
sulphate, it will also liberate an equivalent of copper or |- a 
gram-atom at the kathode and an equivalent of SO4 at the 
anode, and through acidified water it will liberate i gram- 
atom of hydrogen at the kathode and the equivalent | gram- 
atom of oxygen at the anode. This quantity of electricity, 
which is a perfectly definite one, is called a "Faraday " or " farad," 
after the discoverer of the law.^ 

Lonisation. — A curious apparent deviation from the law of 
osmotic pressure is noticed in solutions of salts, acids, and alkalies, 
and indeed of electrolytes generally ; thus a dilute solution of 
sodium chloride produces an osmotic pressure nearly double that 
corresponding to the number of molecules of NaCl present, 
and in fact behaves as if it were a solution of Na and CI existing 
separately. Such a solution conducts a current of electricity 
very readily, while at the same time the chlorine is carried to 
the anode and the sodium to the kathode, where they separate 
as ordinary molecules of Nag and Clg (the Na decomposing the 
water present and forming NaOH). In fact, the modern theory 
of electrolysis asserts that these dissociated atoms are not sepa- 
rated from each other by electricity, but that they exist already 
separated in the solution of the electrolyte, and merely act as 
carriers for the electricity, and that the work done by the latter 
is not that of breaking up the salt-molecule, but of giving its 
dissociated atoms charges of electricity which enable them to 
combine as new molecules and escape from the electrolyte. Com- 

^ It is better to avoid the terms " positive " and " negative " and sub- 
stitute "anode" and "kathode," as the pole connected with the positive 
of the battery is really the negative of the decomposition-cell, and, as will 
be seen below, the electric current is probably a flow of negative electrons 
passing into the kathode, and out of the anode. 

/ 7 



98 PRINCIPLES OF LEATHER MANUFACTURE 

plex salts do not always break up into single atoms ; thus calcium 
sulphate dissociates into Ca and SO4, hydrogen sulphate (sul- 
phuric acid) into 2H and SO4, and so on. These dissociated atoms 
and atom-groups are called " ions," and may be monovalent, 
divalent, or polyvalent, carrying electrical quantities or charges 
in proportion to their valency. Without discussing the ultimate 
nature of electricity itself, the matter is most easily pictured by 
assuming that the molecule of the undissolved salt is made up of 
an ion with a -|- charge (" kation," e.g. Na) and an ion with a — 
charge (" anion," e.g. CI), by the electrical attraction of which 
charges they are held together. In the ionising solution these 
attractions are balanced by those of other ions, and perhaps 
by those of the solvent itself, so that they can wander freely 
within the liquid, but in order to take the molecular form of .free 
elements and escape, say as Na^ and Clg, the pair of kations must 
go to the — pole or " kathode " and give up one -4- charge, and 
at the same time a pair of anions must go to the -|- pole or 
" anode " and receive a -f- charge. Thus the Na and all 
other kations separate at the kathode, and the CI and all other 
anions at the anode. The more modern view is that the kations 
are positive because they are short of a negative (— ) electron, 
and the anions negative because they have one in excess ; the 
galvanic current being therefore a stream of — electrons, flowing 
in at the kathode, and passing out at the anode to combine with 
positive ions. 

From what has been said, it will be obvious that free ions can 
only exist in solution, and can neither evaporate nor separate as 
solids ; but that in the liquid they act much like other dissolved 
molecules, exerting their own osmotic pressure independently of 
each other or of the dissolved salt, but with the limitation that 
the solution must always contain equal numbers of -f and — ions. 

Electrolytes vary much in their tendency to ionise, and even the 
best of them are only completely ionised in infinitely dilute solu- 
tions, but the ionisation always increases with dilution. Just as we 
have spoken of solution-pressure, we may also speak of ionisation- 
pressure, since the ionisation is also an equilibrium determined 
by the point at which as many molecules break up into ions as 
those which recombine, and if the ionic concentration exceeds 
this point, ions recombine till the equilibrium is restored. This 
point has been determined for a large number of substances, and 
is called the " ionisation-constant." " Strong " acids and bases 
are those in which it is large, " weak " ones those in which it is 
very small. Thus for acetic acid it is only •000018, that is, 
acetic acid is only half ionised at •000036 normal, while a 



r 
PHYSICAL CHEMISTRY IN TANNING 99 

" strong" acid like hydrochloric or sulphuric is largely ionised even 
at normal concentration. Ammonia is about as weak a base as 
acetic is an acid, while soda, potash, and lime are strong. The 
salts however, even of weak acids and bases, are largely ionised.^ 
The following table gives the ionisation-constants of various 
weak acids and bases at 25° C. : — 

Ammonia /%=o-oooo23 ' 

Acetic acid ^=0-000018 

Lactic acid ^=0-000138 

Formic acid ^=0-000214 

Gallic acid ^=0-00004 

Boracic acid ^=0-00000001 

Carbonic acid ^=0-0000003 

Butyric acid ^=0-0000115 

Oxalic acid y%=o-ooi 

The Mass-law. — It has been known from the earliest times that 
a " strong " acid would decompose the salt of a " weak " one, 
setting free the latter, but the cause of this was ill understood, 
and was attributed to" a greater affinity for the base. It is now 
known to depend simply on the quantity or concentration of the 
ions, between which, and not wifti the unionised acids and bases, 
most chemical reactions take place. In a mixture of equal parts 
of normal hydrochloric and acetic acids the molecular quantities 
of each acid will be the same, but for every free acet-ion there 
will be many thousand chlor-ions, and consequently the chances 
of meeting an ion of base, if any be present, will be many 
thousand to one in favour of the chlor-ions. Similarly, if hydro- 
chloric acid be added to a solution of sodium acetate, acetic acid 
will be set free, but, instead of remaining in the ionised state, it 
will at once combine with H ions to form the scarcely ionised 
acetic acid, still leaving the CI ions in enormous excess, and 
allowing more acetic acid to be liberated, till its place has practi- 
cally been entirely taken by the hydrochloric. A point more 
difficult to explain, but of great practical importance, is that the 
neutral salt of a weak acid added to the solution of the acid 

1 The ionisation-constants are generally determined by their electric 
conductivity at various concentrations as compared to that at infinite 
dilution, as the electric conductivity varies with the degree of ionisation. 
Ostwald's dilution-formula gives the ionisation of weak acids and bases 
as varied by dilution, but does not apply accurately to strong ones. If k is 
the ionisation constant, v the volume of dilution, and m the proportion 

of ionised acid to the whole, ■v=k. This formula is of wide use in 

(i— m) 

calculations. The ionisation-constant is numerically half the concentra- 
tion at which the substance is half -ionised. 



100 PRINCIPLES OF LEATHER MANUFACTURE 

weakens it still further. The salt, say sodium acetate, ionises 
much more freely than the acid, yielding sodium-ions and acet- 
ions, and the latter, added to those from the acetic acid, increases 
the acet-ion concentration beyond the very low ionising pressure 
of acetic acid, and compels the acet-ions to recombine to unionised' 
acid. Thus by adding sodium acetate to acetic acid, or ammo- 
nium chloride or sulphate to ammonia, we can reduce their 
active acidity or alkalinity to any desired degree without lessen- 
ing their power of combining with and neutralising bases or acids. 
This has many uses in leather manufacture ; thus in deliming 
with an a.cid, we can sufficiently reduce its activity to prevent 
swelling; and in Stiasny's method of "neutralising" chrome 
leather, the alkalinity of ammonia is so reduced by the presence 
of ammonium sulphate, that it is impossible to carry the neutrali- 
sation too far. The presence 'of neutral salts of strong acids 
and bases has, however, little effect in reducing the activity of 
the stronger acids and bases. ^ 

As the difference between the " strength " of an acid or alkali 
and its combining power for bases (or acids), which is what is 
determined by titration, may still not be quite clear to the 
reader, and as it is one of th^ greatest importance in leather 
manufacture, it may be well to give yet another illustration from 
practice. The normal solution of any acid (i eq. per liter) will 
neutralise and combine with an equal quantity of the normal 
solution of any base, but may be very different in sourness or 
apparent alkalinity, as these properties are due to the free ions of 
H+ and OH' which are liberated at the same time as the acid 
or basic ions, and it is the H and OH ions which cause the swelling 
of hide. Lime and caustic soda are almost equally strong alkalies, 
but we can safely put a barrowful of lime into a lime-pit, while the 
same quantity of caustic soda would dissolve the hides. We all 
know why this is ; but little of the lime dissolves, and most 
remains at the bottom of the pit, and goes gradually into solution 
to strengthen the liquor as it is taken up by the hides. Almost 
the same explanation, with a difference, applies to hydrochloric 
and acetic acids, though both apparently dissolve at once. The 
hydrochloric acid suffers almost completely the sort of super- 

1 The mathematical statement is that if a and b are the two ions of 
which a salt (or acid or base) c is composed, and k is its ionisation-constant, 
axb=kc, from which it is obvious that the increase of either of the two 
ions must increase the concentration of the unionised c, as A is a constant. 
A simple illustration is that a party of either girls or men coming to a 
dance will increase the number of dancing couples (combined molecules !) 
by taking partners from those " sitting out." Acids are salts of hydrogen 
with an acid-ion ; alkalies, salts of hydroxyl (OH) with a base. 



PHYSICAL CHEMISTRY IN TANNING loi 

solution which we call ionisation, and exerts its full strength 
immediately, while the acetic acid, like the lime, remains in the 
liquor mostly in an inactive unionised state, and only ionises when 
the already ionised part has been consumed by the hide. 

As the actual ionic concentration is the important point in 
many technical processes, and cannot be determined by titration, 
but only by electrometric measurement, or the gradual change 
of colour of indicators {cp. p. 103), it will be convenient to speak 
of it as the " true " acidity or alkalinity of a solution, as dis- 
tinct from that shown by titration. 

A" very interesting example of equilibria is given by an ordinary 
method of preparing pure common salt. Hydrochloric acid is 
even more highly ionised than salt, so that if the concentrated 
acid is added to a saturated salt solution a part of the salt is pre- 
cipitated. In a saturated solution of sodium chloride with solid 
salt present we have dissolved salt at the solution-pressure of the 
crystallised salt, and Na and CI ions at the dissociation-pressure 
of the saturated salt solution, and neither affect the others. If 
we now add hydrochloric acid, it has no effect directly on the 
solubility of the salt, but as HCl dissociates largely into H and CI, 
it increases the pressure of the CI ions, and so compels the salt 
to recombine till the CI pressure is reduced to its normal amount. 
This increases the concentration of the undissociated salt solution, 
and thus salt is precipitated or crystallises out till the solution 
is no longer supersaturated with respect to the salt -crystals. 

The neutralisation of acids by alkalies in solution is really the 
combination of the H and OH ions to form water, and hence the 
heat of the reaction is practically the same for all acids and 
alkalies. The saltions remain ionised, and only actually com- 
bine when the solution is concentrated. 

Salt-hydrolysis. — Water ionises to a very minute extent into 
H+ and OH', only about o-i grm. per ton being ionised, and its 
ionisation-constant is 1-2 X io~^*, or slightly over one ten-millionth 
normal. It might be supposed that so small an ionisation might 
be neglected, but in the case of salts of acids or bases of which 
the ionisation constants are so low as to approach this figure 
the results are very important. Carbonic acid; for instance, is 
such an acid, and many of the amino-acids present in proteins 
come into the same category, though their salts with strong bases 
ionise freely. Sodium carbonate thus ionises into Na+ and CO3". 
When the CO3" comes in contact with the H+ ion of the water it 
at once combines with it to form almost unionised HgCOg, and 
so goes out of the equilibrium, and further water is ionised to 
restore its H+ concentration to the old amount, and this process 



102 PRINCIPLES OF LEATHER MANUFACTURE 

is repeated till the acid reaches such a concentration that its 
ionisation is equal to that of the water. During this process 
the OH and Na ions accumulate in electric equilibrium, and as 
alkalinity is due to the OH ions, the solution becomes strongly 
alkaline. A similar result occurs with a little ionised base such 
as alumina and a strong acid ; the H ions accumulate, and the 
solution becomes strongly acid, as in the case of aluminium 
sulphate, although in both cases the salts are chemically normal. 
The salts of " strong " acids with " strong " bases are not 
perceptibly hydrolysed, and so whether methyl orange or phenol- 
phthalein be used as indicator, the change of colour is practically 
simultaneous, and so soon as sufficient acid is added to neutralise 
the base the solution becomes instantly alkaline, but with 
" weak " acids or bases the change of colour is very gradual, 
and there is a wide interval between the two indicators, which in 
former times was supposed to show that the combination was 
not "chemical," and that there was no definite point of saturation; 
or such ideas as that of a series of valencies successively neutralised 
was adopted to account for it. The simple reason is that owing 
to salt-hydrolysis ^ the increase of H+ or OH' concentration is 

^ The amount of hydrolysis may be deduced from the mass-law and the 
ionisation-constants of the substances concerned, but except for mono- 
valent acids and bases, only one of which is weak, it becomes very com- 
plicated, and the reader is referred to the larger books on physical chemistry. 
Where both acid and base are weak the amount of hydrolysis is not 
affected by dilution, but in the simpler case of only one weak monovalent 
acid or base it follows the same dilution law as ionisation (p. gg), which 

may be expressed by the equation k' = i jv, where v is the volume, 

m the proportion of hydrolysis to the whole, which is taken as i, and k' 

a hydrolysis-constant, different from the ionisation-constant k, but bearing 

k 
the relation to it that k' = — , where k is the ionisation-constant of the weak 

kw 

acid or base, and kw that of water, i-2Xio-i*. Thus, if the hydrolysis- 
constant is known, the ionisation-constant can be calculated from it, and 

vice versa. {Cp. p, g8.) For many purposes the simple formula, w = — -—;, 

X 

or I —m = r, where m is the proportion hydrolysed (the whole being taken 

X -p R 

as unit}^), x the hydrion- or other ionic-concentration, and k either the 
ionisation- or the hydrolysis-constant as may be required, is more con- 
venient, and is merely another form of the same equation. {Cp. App. B.) 
The curve rises rapidly at first, and gradually becomes horizontal, only 
reaching the value of unity at infinite concentration, but if the hydrolysis- 
constant is known, unity can be calculated, or very approximately esti- 
mated, by plotting on curve paper. If the constant is correct, unity is 
the equivalent combining weight of the substance. 



PHYSICAL CHEMISTRY IN TANNING 103 

a very gradual one, and as each indicator changes colour more 
or less gradually over a certain range of H+ concentration, there 
is often no definite end-point. Phenolphthalein changes slightly 
on the alkaline side of true neutrality, methyl orange on the 
acid side, and thus there is a considerable range of hydrion 
concentration between the two. Phenolphthalein is therefore 
suitable for the titration of weak acids, since it does not react 
till their neutral point is actually passed, and methyl orange for 
a similar reason for weak bases. 

The Determination of Hydr ion-concentration. — It has been shown 
(p. 100) that mere titration furnishes no guide, or at most a rough 
one, to the " true acidity," which depends on the actual hydrion- 
concentration, and which is really the cause and measure of most 
of the changes which take place in tanning. There are at present 
only two methods which are available for this purpose ; the 
electrometric, which, though very exact, requires complicated and 
expensive apparatus and considerable skill in its use, and the 
colorimetric, which, though less exact, is sufficiently so for most 
technical purposes, and is easy and inexpensive, and will therefore 
be first described. It has been mentioned in the last section that 
different dyestuffs used as indicators change colour at different 
hydrion concentrations, and do so gradually over a certain 
small range, so that by suitable choice the whole range of the 
small concentrations where hydrion-determination is important 
can be covered. For this purpose an indicator is often most useful 
which does not change too suddenly but covers a considerable 
range of concentration, while for an ordinary indicator in titration 
the change cannot be too rapid. It is obvious that in a colourless 
liquid all that is necessary is to place the liquid to be measured, 
and another of known hydrion-concentration, in two similar tubes 
with equal quantities of the selected indicator, and vary the known 
concentration till the two exactly match. In practice it is found 
more convenient, and usually sufficiently accurate, to have a 
series of, say, ten liquids of graduated and known hydrion-con- 
centration, and to- compare these successively till a match is 
made, or a pair found between which the tested liquid can be 
placed. Where it is required, as will often be the case, to bring 
a liquor to a definite degree of true acidity, this can be done by 
gradually acidifying the liquor with some weak acid till it matches 
the standard tube, and adding the same proportion of acid to 
that in use on the large scale. Unfortunately tannery liquids 
are rarely colourless, which has proved a bar to the use of the 
method until the invention of the " Comparator " by Walpole,^ 
^ See Appendix D. 



104 PRINCIPLES OF LEATHER MANUFACTURE 

which overcomes the difficulty by a very simple means. Instead 
of two tubes, four are used in pairs one behind the other in front 
of a suitable light (often a Welsbach gas-burner). In the two 
front tubes the liquor to be tested, with its indicator, and the 
standard colour liquid are placed, while behind the latter is a tube 
of the unaltered liquor, and behind the liquor-tube with the 
indicator is a tube of pure water, so that the colour of the liquor 
is exactly balanced. 

With regard to the electrometric method, only the principle 
can be explained here, and for working details the reader is referred 
to other books. ^ 

If we imagine two galvanic cells, both alike, say consisting of 
copper in saturated copper sulphate, and zinc in zinc sulphate 
solutions of equal concentration, with their positive poles opposed 
to each other, and their negative also connected, there wiU be 
obviously no current produced, but if the zinc sulphate solutions 
are of unequal concentration, a current wiU be set up in such 
a sense than zinc will be dissolved in the weaker solution till its 
osmotic pressure and concentration becomes equal to the other, 
that is, a positive current will pass from the anode of the weaker 
to that of the stronger cell until equilibrium is restored. It is 
obvious that the energy which produces this current is the 
difference of ionic osmotic pressures, and as the relation 
between these two forms of energy is known, the electric pressure 
or " potential " can be calculated from the ionic concentration, 
or vice versa. 

In our particular case it is the ionic concentration of the H+ 
ion which we have to determine, and clearly if we could use a 
plate of metallic H in place of the Zn, we could accomplish it by 
a similar arrangement. This cannot of course literally be done, 
but it is found that if we bubble hydrogen gas over a plate of 
platinum coated with platinum black, enough gas is absorbed 

^ Books of reference as to electrometric methods. Ostwald-Luther for 
practical details of measurement. J.S.C.I., 30, 191 1, p. 3, Wood, Sand, 
and Law describe the use of a very convenient but somewhat expensive 
apparatus designed by Dr Sand for the determination of H+ concentration 
in puers and tan liquors. The same apparatus is figured and described in 
Wood's book on Puering, Bating, and Drenching of Skins (Spons, 1912). 
The simple bridge-wire method described in Ostwald-Luther is at least 
equally accurate, but not being self-contained, is not quite so convenient 
to handle. Before the war good and cheap apparatus could be got of 
Kohler, Leipzig. Sand's apparatus is made by Griffins. An excellent 
discussion of the subject, and many valuable hints on practical work, and 
also on the use of indicators, may be found in Sorensen's Comptes-rendus 
du Laboratoire de Carlsberg, 8, 1909 (Hagerup, Copenhagen). 



PHYSICAL CHEMISTRY IN TANNING 105 

to make it behave like one. In place of the copper and copper 
sulphate we use mercury and a solution of potassium chloride 
mixed with powdered HgCl (calomel), which is very difficultly 
soluble, and keeps the solution saturated, as it is found that this 
arrangement gives a still more constant electromotive force than 
the copper. If we imagine a pair of such galvanic elements 
opposed to each other, each composed of a similar mercury 
electrode and a hydrogenised platinum plate immersed in an 
acidified solution, we can see that these two solutions must be of 
exactly equal acidity (hydrogen-ion-concentration) if no current is 
to be produced. If instead of adjusting the solutions we measure 
the potential (or electric pressure) between the two cells which 
tends to produce a current, employing a known hydrion solution 
in one cell, we can calculate the concentration in the other. 

A still simpler way is to measure the potential produced by a 
single element consisting of a calomel electrode and a hydro- 
genised platinum plate in the acid solution, having previously 
determined the potential produced with a normal acid solution, 
and this is the method now generally used. Sorensen has found 
that the potential of such an element with decinormal potassium 
chloride solution and calomel in the negative, and acid, normal 
with regard to the ionised hydrogen in the positive cell (corre- 
sponding to Ph = on the Sorensen scale), is 0-3377 volt, and as 
the potential rises with the dilution of the hydrion solution, the 
potential for any given dilution is Pot. =0-3377 + 0-0577 xPh, 

T _ Pot. —0-3377 ^ 00 /- 

or conversely, P„= ^^^^^ at 18 C. 

0-0577 

The Logarithmic Expression of Numbers. — For the expression 
of ordinary numbers, such as we use in everyday life, the ordinary 
Arabic notation is sufficiently convenient, but when we come to 
deal with very large or very small numbers, as is frequently 
necessary in science, it becomes very cumbrous. It is much 
shorter to write £4 x 10^ than £4,000,000,000, and it means 
quite the same thing, and if we are to accumulate debt at our 
present rate we may have to adopt it ! The explanation of the 
expression is quite simple — 10 multiplied once by itself (its 2nd 
power) is 100 ; 10 multiplied six times by itself is a million, and 
so on, and the exponent 9 used in the illustration means that 
nine o's have to be affixed to i to express it in the ordinary way. 
What is rather more puzzling to the non-mathematical is the 
expression lo^^, but it is really equally simple. A minus ex- 
ponent is a reciprocal ; thus io~^ is or o-ooooooooi, 

looooooooo 



io6 PRINCIPLES OF LEATHER MANUFACTURE 

that is, the i (or the first significant figure whatever it may be) is 
in the gth place of decimals. 

In Sorensen's exponential or logarithmic notation, Ph =3 means 
that the divisor of a solution normal as regards hydrogen -ions, 
or containing i eq. = i grm. of ionised hydrogen per liter, is 10^ 
or 1000, or that the solution is N/iooo, and so on. As, however, 
it is inconvenient to have to prefix a multiplier, say 1-5 xio^ to 
signify a N/1500 solution, the fractional power of 10, which equals 
1-5, or, in other words, the Briggsian or common logarithm, is 
used, and we write Ph=3-i8, o-i8 being the log. of 1-5. Thus the 
exponent of the 10 is simply the index of the log., and the decimals 
are the mantissa. 

The following short table giving the number from i to 10 and 
the mantissae of the corresponding logarithms will be sufficient 
for the conversion of the Sorensen exponential scale into the 
divisors of normality : — 

Number 123456789 10 
Logarithm o-oo 0-30 0-48 0'6o 070 0-78 0-85 o^go 0-95 i-oo 

Hydrion-concentrations. — The concentrations of both hydrions 
and hydroxyl-ions in pure water is about i-i x io~' at 18°, and on 
the exponential scale ?„ and Pqh each equal 7-07, and this is 
the point of true neutrality, and solutions of a greater Pjj are 
alkaline. The ionisation-constant of water ky, is the product of 
these, and about i-2Xio~^^, or on the exponential scale 14-14, 
since the addition of exponents or logarithms is equivalent to the 
multiplication of their numbers ; and this product is constant 
in water and all dilute solutions. Consequently if we raise the 
concentration of hydrions to normality by the addition of acid, 
that of hydroxyl-ions will fall to zero, and vice versa if we add 
alkali. Since the determination of hydrions is much easier and 
more exact than that of hydroxyl-ions, it is customary to express 
both acidity and alkalinity as hydrion-concentration, but the value 
of Pqjj can always be found by subtracting Pu from 14-14 (or for 
ordinary purposes from 14). The convenience of the exponential 
scale is obvious in practice, since, when we come to very great 
dilutions, it is rare that we can attain to much greater accuracy 
than to say M'hether we are dealing with millionth or 10 millionth 
dilution. 



CHAPTER X 

COLLOID SOLUTIONS AND THE COLLOID STATE 

The Colloid State. — The term " Colloid " (signifying " gluey ") 
was first applied by Graham to the class of bodies of which glue 
and gum are typical examples, and of which the solutions have no 
definite point of saturation, and have little, if any, tendency to 
diffusion or osmotic pressure. It has since been extended to 
include what were earlier called " pseudo-solutions," in which the 
dissolved substance is contained, not as molecules, but as ex- 
tremely small particles permanently suspended in a liquid. It 
has been more recently shown by von Weimarn, Hopkins, and 
others that many typical colloids, such as egg-albumin and 
gelatin, can be crystallised, while such definitely crystalline 
substances as even common salt can be obtained in the colloidal 
form by precipitation in liquids in which they are sufficiently 
insoluble, so that it is now more proper to speak of the " colloid 
state " than of colloids as a separate class. 

The pseudo-solutions, now more frequently called " suspension- 
colloids," or simply " sols," concern us principally because basic 
chrome, iron, and alumina solutions and basic salts generally are 
possibly sols of the respective oxides in the solution of normal 
salt, though this point requires further investigation. Sols may 
be obtained in a variety of ways : by precipitation in liquids in 
which the precipitate is extremely insoluble, by " peptisation " 
or the bringing of precipitates already formed into the sol con- 
dition, or by the electric spraying or electrolysis of metals, but 
the essential point seems to be their insolubility in the liquid 
medium, as otherwise the smaller particles would dissolve and 
recrystallise on the larger ones, and the solution would be pre- 
cipitated. Very typical are the colloid solutions of metallic 
gold obtained by the reduction of ver}^ dilute solutions of gold 
chloride by substances which do not at the same time form 
electrolytes in the liquid. These sols are red or deep purple, 
and perfectly transparent, and pass through filter-paper like true 
solutions, but they can be shown by the " ultra-microscope " 
of Zsigmondy to be coloured by actual particles of metallic gold, 
of extreme smallness, and in rapid vibratory (Brownian) motion, 

107 



io8 PRINCIPLES OF LEATHER MANUFACTURE 

caused by the collisions of the molecules of the solvent in their 
heat-vibrations.^ 

The suspended particles, though in constant motion, are pre- 
vented from actual contact by electric charges, which, being of 
the same sign, cause them to repel each other and prevent their 
coalescence, but the addition of an electrolyte such as common 
salt to the solution causes their discharge, and the sol coagulates 
and precipitates. This is probably due to the electric adsorption 
of the ion of opposite charge to that of the particle, and as a rule 
the higher the valency of this ion the greater the precipitating 
effect. Additions of very small quantities of certain colloids, 
mostly proteins, greatly lessen, or even prevent, this precipitation, 
and are hence called " protective colloids." Of these gelatin 
is perhaps the most powerful, i mgr. protecting a liter of gold- 
sol from the precipitating effect of a large quantity of salt, 
probably by coating the particles. The precipitates from sols are 
often called " gels," but the term must not in any way be con- 
founded with jellies, which have quite different properties. The 
addition of small quantities of protective colloids greatly favours 
the formation of colloid sols, and the presence of gelatin has this 
effect in the preparation of photographic emulsions. 

While electrolytes in quantity rapidly precipitate sols, the 
presence of traces seems essential to their formation, probably 
because ions are necessary to give the electric charge. A colloid 
solution of iron oxide in water is obtained by dialysing ferric 
chloride through a parchment-paper membrane, through which 
the iron oxide will not pass, while the hydrochloric acid produced 
by the hydrolysis readily does so, but however long the process 
is continued, a trace of CI is always found in the colloid solution. 
This solution has been used as a tonic under the name of " Fer 
Bravais," and is quite clear, and of a dark brown colour, but 

^ The ultra-microscope in its original, and still most perfect, form con- 
sisted of a glass-sided cell on the stage of the microscope, the liquid in which 
was brilliantly illuminated by a thin, flat horizontal beam, concentrated 
on it by a powerful and accurately adjusted condenser. As the thickness 
of the beam was exactly known, it was possible to count the particles in a 
known volume of liquid. A nauch simpler, but less quantitative method 
is to concentrate the light in the liquid under a cover-glass on an ordinary 
slide by a substage condenser of very wide angle, so that the brilliantly 
illuminated particles are seen on a perfectly dark ground, since the centre 
of the condenser is darkened, and the light is projected from all sides at 
such an angle that it cannot pass through the cover-glass on account of 
total reflection. Probably the best of these is the Zeiss " cardioid con- 
denser," but for many purposes a good " paraboloid " answers well, and 
is somewhat easier to manipulate. The illumination of ordinary dark- 
ground condensers is insufficient. 



COLLOID SOLUTIONS AND COLLOID STATE 109 

is extremely unstable, being precipitated immediately on heating, 
or even on dilution. It is highly probable that colloid solutions 
of alumina and chrome oxide might be produced by similar means, 
and the subject will be again alluded to in dealing with chrome 
and with the theory of tannage. 

Mutual Precipitation of oppositely charged Colloids. — It has 
been stated that the colloid particles have electric charges, prob- 
ably from attached ions, and that the repulsion of these similar 
charges prevents their contact. If, however, a colloid sol of + 
charge be mixed with one of — charge, the particles attract each 
other and produce mutual precipitation, and if the quantities 
are electrically equivalent, the precipitation may be quite com- 
plete, but if one or other is in excess, neither is usually completely 
separated. It is probable that the precipitation of gelatin by 
tannin is an instance, and in this case, if the gelatin be in excess, 
a portion of the tannin remains dissolved. Many dyestuffs are 
colloidal, and we know that basic dyes are constantly precipitated - 
by acid ones. It is hard to make any distinction between such 
reactions and those which are strictly chemical, since both are 
due to the combination of + and — ions, but the colloidal charges 
vary with the size of the particles, and thus are less definite than 
those of single ions. 

Kataphoresis and Electric Osmose.- — If an electric current be 
passed through a colloid sol, the — charged particles will move 
towards the -]- pole or anode, and the + charged towards the — 
pole or kathode, that is to say, towards a region of higher electric 
charge of the sign opposite to their own. Arrived at the pole they 
usually become discharged, and often form a clot of precipitate, 
but cases are known where they take fresh charges of the opposite 
sign and return towards the other pole, sometimes meeting and 
precipitating oppositely charged particles midway. The particles 
thus behave precisely like the ions in electrolysis, and of course 
like them possess an electrolytic conductivity, though a small one. 
Electric osmose is the converse of kataphoresis. If a cell is 
divided into two compartments by a porous partition with an 
electrically charged surface (as, for instance, earthenware, 
powdered glass, bladder, or parchment paper) and a current be 
passed through, the liquid will flow through the partition in the 
opposite direction to that in which the material of the partition 
would have moved if it were in the form of suspended particles, 
and if it cannot escape, will rise considerably above the level of 
the side from which it flows. This effect explains the theory of 
electric tanning so far as it has one. If an alternating current 
is used there will be no steady flow, but merely a vibration ; 



no PRINCIPLES OF LEATHER MANUFACTURE 

and if a continuous, the process is complicated by electrolytic 
decomposition, and indirect destruction of tannin. 

Kataphoresis is important as being the simplest mode of 
determining the sign of the charge on suspended particles. The 
experiment is usually made in a U-tube. If the particles are not 
visible to the eye, their concentration in the two limbs of the tube 
is determined by analysis. The following table gives the sign 
of the charge on a few of the commoner colloids. The charge 
is often reversed in acid and alkaline solutions, and with ampho- 
teric bodies changes at the isoelectric point (see p. iii). 

Positive Solutions Negative Solutions 

(to kathode). (to anode). 

Hydroxides (of Fe, Cr, Al, Metals (x\u, Ag, Pt, Pd, Ir, Cd). 

Cu, etc.). Metallic sulphides. 

Some metals, Pb, Bi, Fe Silver halides. 

(probably hydroxides in water). Proteins in alkaline solution. 
Proteins in acid solution. Silicic acid in alkaline solution. 

Silicic acid in acid solution. Starch, mastic, caramel, resin, 

shellac. 
Iodine, sulphur, selenium, tel- 
lurium. 
Oil emulsions. 

Natural Organic Colloids. — These, proteins, gums and the like, 
are now frequently styled " emulsion-colloids," on the assump- 
tion, by no means proved, that their apparent solutions are fine 
emulsions of one liquid in another. They have many analogies 
with the suspension-colloids, but also marked differences, and it 
is not unlikely that their colloid character arises from quite 
different causes. They all have large molecular weights, and 
mostly a tendency to polymerisation (the grouping of several 
or many molecules together), and their solutions have large 
viscosity, and not unfrequently take the form of consistent jellies • 
(not gels). Their solutions are not precipitated by small 
quantities of electrolytes, and the " salting out " which occurs 
when some salts are added in large quantities is probably quite a 
different phenomenon. Their solutions generally show at least 
traces of reflected light (Tyndall effect) when a powerful beam is 
passed through them, but it is only rarely that separate particles 
can be detected by the ultra-microscope, and these are possibly 
accidental impurities. Many of them, especially the proteids, 
are amphoteric, or capable of acting both as acids and bases, as 
will be readily understood from their chemical constitution 
(see Chapter XI.). Their molecules or particles possess electric 



COLLOID SOLUTIONS AND COLLOID STATE iii 

charges, but there is a point, corresponding to a definite alkahnity 
or acidity (p. ii8), at which they are neutral, and on the acid side 
of which they behave as bases and on the alkaline as acids, but 
never both at once. This is called the " isoelectric point," and 
is not usually exactly at the neutral point of water. Generally 
this point is also that of minimum swelling. 

Viscosity is so important a characteristic of these solutions that 
it demands special consideration. The viscosity of oils, glycerine, 
and more or less of all liquids, is simply internal liquid friction 
impeding their rate of flow, and is usually, and properly, measured 
by the time required for a measured volume to pass through a 
capillary tube under a definite pressure. This is the construction 
of all ordinary viscosimeters, and is quite satisfactory even for 
gelatin solutions above the melting point of their jellies, but as 
this is approached and passed, even very dilute solutions, ap- 
parently quite liquid, suffer a rapid increase in viscosity, which is 
apparently of a different character, and arises from j elly-structure. 
Such a solution, if passed through a capillary viscosimeter a 
second time, always shows a lower viscosity than at first, and the 
viscosity is also much lowered by shaking or violent agitation, 
pointing to the breaking up of some structure {cp. also p. 113). 
A noteworthy fact about viscosity of the proteids is, that with 
increase of acidity it usually rises to a maximum and then again 
falls, and that the maximum appears to be at approximately 
the same ?„ as that of maximum swelling, where this has been 
determined by other methods, while it is at a minimum at the 
isoelectric point. -^ 

Dialysis and Ultra-filtration. — Colloids will not, as a rule, pass 
through colloidal membranes, such as parchment paper, bladder- 
or gold-beaters'-skin, or through thin films of gelatin jelly, 
or filter-paper soaked in gelatine solutions, while crystalloids 
generally pass freely. This affords a means of separating colloids 
from crystalloid impurities. The colloid solution is, for instance, 
placed in a parchment-paper drum and floated on water, which 
is frequently renewed. Jellies form their own diatyser. Swollen 
gelatin which has been treated with a weak acid to bring calcium 
sulphites and phosphates into solution can be freed from them by 
washing on a canvas-bottomed tray floated in water. 

Bechhold's ^ method of ultra-filtration is filtration under slight 
pressure through filter-paper which has been impregnated with a 
porous film of collodion or gelatin. The degree of porosity can 
be regulated by varying the strength of the collodion or gelatin 

^ Loeb, Journ. Gen. Physiol., 1918-19, vol. i., pp. 39, 237, 363. 
^ Zeit. Phys. Chem., 60 (1907), p. 257, and 64 (1908), p. 328. 



112 PRINCIPLES OF LEATHER MANUFACTURE 

solution, so as to allow some colloid particles to pass while retain- 
ing others. For details the original paper must be consulted. 

Fractional precipitation is another means which can sometimes 
be employed with success for the separation and purification of 
colloid solutions. Sulphates of ammonia or soda are frequently 
employed for this purpose. If added in sufficient quantity the 
whole of the really colloidal constituents may be flocculated 
and separated, generally carrying down with them some non- 
colloidal matter, which may be removed by re-solution and re- 
precipitation. If the quantity of the precipitant added be in- 
sufficient to precipitate the whole of the colloids, the first 
precipitate will be mainly of those most easily dehydrated, 
while those with a greater attraction for water can be brought 
down by a further addition of the salt. Of course neither of 
these fractions is a pure individual proteid, but this may be 
approached by repeating the operation, perhaps with a less or 
different addition of the salt. The precipitation seems to be 
one of simple dehydration, and the salt retained in the precipitate 
may be removed by dialysis. The theory of this " salting out " 
is somewhat uncertain. It is possible that it is due to the hydra- 
tion of ions withdrawing the water from its function as a solvent, 
or that the jellies, though permeable to the ions, are not so to 
the unionised salts, the osmotic pressure of which therefore 
expresses the water from them, just as in the case of dehydration 
by alcohol, which does not ionise to any extent. Proteins are 
usually insoluble in water at the isoelectric point. 

Jellies. — Many organic colloids remain in more or less viscous 
solution even at low temperatures, but others, including gelatin, 
set to a more or less coherent jelly on cooling. Much discussion 
has taken place as to the physical structure of these jellies, the 
earlier school of colloid chemists holding with Biitschli and van 
Bemmelen that they had a sponge-like structure with open pores 
which might even be made visible under the microscope, while 
the adherents of the emulsion-theory of organic colloids have 
held that they consisted of globules containing water (or some 
other solvent) separated by a liquid consisting of a weaker colloid 
solution. It has, however, become evident, especially since the 
work of the writer on the gelatin equilibrium with acids, that 
neither of these theories can be maintained, at least in their 
original form, and that the net-like structure, if it exists, cannot 
be of more than molecular dimensions, since the whole mass is in 
chemical equilibrium with the surrounding solution, and must 
therefore be within the sphere of molecular attractions. The 
prevailing idea at present seems to be that of a network of mole- 



COLLOID SOLUTIONS AND COLLOID STATE 113 

cules, possibly crystalline. From Chapter XI. it will be seen that 
these molecules are probably long chains of amino-acids, which 
would lend themselves to such a structure. Jellies also present 
striking analogies in their behaviour with external solution to 
that of two immiscible solvents with a third substance soluble 
in both, and probably the safest position is to regard them as 
solid, or semi-solid solutions without theorising too much at 
present as to their ultimate structure. ■*■ 

In the following pages attention will be particularly directed to 
the gelatine jelly, not only because it specially interests us by its 
close relation to skin, but because it is typical of most other 
gelatinising colloids. A solution of gelatine heated above 70° C. 
undergoes chemical changes which gradually diminish its setting 
power, which after continued boiling entirely disappears, but 
below 70° the change from solid to liquid seems quite rever- 
sible. A solution of good gelatine at 70° shows few or no ultra- 
microscopic particles, and only a very slight Tyndall effect, 
and may well be an actual solution of large molecules. As it 
cools, both the Tyndall effect and the viscosity increase, but only 
slowly, owing to the sluggish motion of the particles, and it is 
often some days before the full effect of a given temperature is 
reached. As the temperature nears the setting point the rise 
of viscosity is much greater, though even after complete setting 
the firmness of the jelly continues to increase for many hours or 
even days. The viscosity near and below the setting temperature 
seems to be of a different type to the ordinary frictional viscosity 
of higher temperatures, and is apparently due to some sort of 
structure, as it is diminished by vigorous stirring, or even by 
passing through an ordinary capillary viscosimeter."^ Solutions 
so dilute as to be apparently quite liquid below the setting point 
still show these effects, and a disc suspended in the liquid by a 
torsion wire will only move a short distance and take up a per- 
manent position at a slight torsion. Such liquids also show the 
dipolarising effects of strain in the polariscope if examined in the 
annular space between a fixed and an interior rotating cylinder 

^ Certain organic substances which ultimately form masses of needle- 
like crystals take at first a jelly-like condition, and even masses of crys- 
talline salts have often a considerable resemblance to jellies. Gelatin 
can be crystallised by very slow evaporation, and some observers have 
noted resemblance to thread-like crystals in the ultra-microscope. Soaps 
form very decided j ellies, but are alkaline salts of fatty acids, and decidedly 
crystalline. 

^ Cp. Arisz, Kolloidchem. Beihefte, 7, i, 1915, long abstract in J.S.L.T.C. 
Arisz describes some curious results in heating and cooling solid jellies, 
but as these contained glycerine, it is difficult to draw conclusions. 



114 PRINCIPLES OF LEATHER MANUFACTURE 

(Kundt, Wied. Ann., 1881, 13, p. no), which is quite absent 
in much more viscous Uquids such as glycerine and sugar 
syrup, though it is shown by fatty oils. 

If we place a sheet of dry gelatine in water it swells, absorb- 
ing perhaps seven or eight times its weight of water, but does 
not appreciably dissolve. A condition of equilibrium is reached 
when the osmotic pressure within the jelly is equal to the sum of 
the cohesive attraction of the gelatine for itself and the osmotic 
pressure of the water outside. An increase of the cohesion of the 
gelatine would tend to make it contract and expel part of the 
water, and this contraction would tend further to increase the 
cohesion of the gelatine. The equilibrium is therefore a very 
unstable one, and slight causes might be expected to produce great 
changes in the degree of swelhng, which is indeed the case. If we 
increase the temperature we diminish the cohesion of the gelatine, 
till at a point it becomes less than its attraction for the water, 
and the jelly suddenly loses its solid condition and dissolves. 
The melting point of good gelatine varies somewhat, but is usually 
between 25° and 30° C. With glues and inferior gelatines it is 
often much lower owing to the presence of gelatoses, and forms a 
useful test of commercial quality. 

The absorption of water by dry colloids (including gelatine) is 
accompanied by contraction of volume (compression of the 
water absorbed or of the colloid itself) and by evolution of heat, 
and, as has been pointed out by Korner,^ it is opposed by increase 
of temperature, but at the same time the cohesion of the jelly is 
decreased, which favours swelling. Solution, on the other hand, 
absorbs heat, and is therefore favoured by rise of temperature. 
The problem is therefore very complex, and the actual result 
probably varies with the degree of hydration of the jelly. 

If we place the swollen jelly in alcohol, it parts with water 
and contracts. The gelatine and alcohol are not mutually 
soluble ; the sum of the attraction of water for alcohol and the 
cohesive attraction of the gelatine is greater than the attraction 
of the latter for water, and as the alcohol cannot pass into the 
gelatine, the water passes out, and the jelly contracts. The 
greater the concentration of the alcohol, the more completely 
is the jelly dehydrated, and in strong alcohol it may become 
quite hard and solid. This method of dehydration was adapted 
by Stiasny for estimating the water content of pelt. If we like to 
express the same facts in language more familiar to the modern 
chemist, but perhaps less clear to the non-chemical reader, we may 
say that the alcohol exerts an osmotic- pressure outside the 
1 Beitrage zur wissenschajtlichen Grundlage der Gerberei, Freiberg, 1899. 



COLLOID SOLUTIONS AND COLLOID STATE 115 

gelatine, but little or none inside it, and therefore the water is 
squeezed out, till the osmotic pressure is equal in both the jelly 
and the alcohol. The jelly is a true " solid solution " of water 
in gelatine ; we may regard either of the two constituents as the 
solvent. Exact parallels may be found in the distribution of a 
third substance between two immiscible solvents (see p. 93), 
say, alcohol between water and benzene. 

The osmotic pressure of alcohol may be demonstrated in 
a very simple way, taking advantage of the fact that a film 
of jelly is permeable for water but not for alcohol. If the 
experiment described on p. 96 be made by placing alcohol in a 
cell previously washed out with a gelatine solution, and the cell 
be placed in water, the water will pass into the cell, and the 
alcoholic solution will rise many feet in the vertical tube. The 
insolubility of gelatine in alcohol may be made use of for its 
estimation. If at least three times its volume of absolute alcohol 
be added to a solution containing gelatine, the latter will separate 
as a solid mass on a stirring rod, or on the sides of the beaker, 
and may be washed with further portions of alcohol. The method 
is useful in the proximate analysis of gelatine lozenges and ' ' j elly 
squares," roller compositions, hectograph masses, and the like, 
and for the determination of true unaltered gelatin in glues and 
commercial gelatines (see p. 150), but it must be remembered 
that many other colloids are also precipitated by alcohol. 

If hide be treated with alcohol, as in Knapp's experiment 
(p. 575), the action is precisely the same as has been described 
with gelatine-jelly. The water is withdrawn, first from the 
spaces between the fibres, and then from the fibres themselves, 
and the skin dries with the fibres isolated and non-adherent, 
and is in fact converted into a sort of leather, which, however, 
returns to raw pelt on soaking in water. 

The action of solutions of sugars, glycerine, and the like is in 
principle similar to that of alcohol, but more complex, since in 
general these bodies are soluble not only in the water, but in the 
gelatine or hide-fibre, so that their effect cannot be foretold, 
though usually it tends towards contraction rather than swelling. 
In general terms the equilibrium is a balance of the attraction of 
the water and the sugar for the gelatine, against the sum of 
their mutual attraction in the solution outside and the resisting 
cohesive force of the gelatine ; and will depend not only on the 
nature of the substances, but on temperature and concentration. 

The action of acids, alkalies, and salts on gelatinous fibre is 
yet more complex, since not only electrolytic dissociation, but 
actual chemical combination comes into the question. The 



ii6 PRINCIPLES OF LEATHER MANUFACTURE 

detailed chemical constitution of gelatine is as yet uncertain, 
but it is known (see Chapter XL) to consist of chains of amino- 
acids, linked by the carboxyl of one to an amino-group of the 
next, leaving an amino-group NH2 at one end of the chain and a 
carboxyl (COOH) at the other. Hence hide-fibre absorbs both 
acids and bases with great avidity, so much so that the sulphuric 
acid of a decinormal solution may be completely removed by hide, 
leaving only water without a trace of acid recognisable by litmus. 
Alkalies are absorbed in a similar way, and in both cases the 
gelatine or gelatinous fibre acquires a greatly increased power 
of absorbing water, and consequently of swelling. Familiar 
cases of this are the swelling of hide by acid and by lime, and in 
neither case can the added substance be removed in any reason- 
able time by mere washing with water. Hence to free hides from 
lime or acids it is necessary to neutralise the alkali with acids 
(see p. 203) or the acid with chalk or alkalies (p. 206). 

If hide or skin, previously swollen with acid, be placed in a 
strong solution of common salt, it is dehydrated and the swelling 
completely reduced, and, as in Knapp's experiment with alcohol, 
it is converted into a sort of white leather, which however 
swells again strongly if placed in water. This is the cause of 
" pickling," which is described in Chapter XV. A similar action 
takes place with acid gelatine, which becomes quite hard and 
horny. 

When dry gelatine is placed in water, it absorbs it with great 
avidity at first, but afterwards more slowly, forming a jelly. 
Dry hide behaves in the same way (though the jelly is one of 
collagen, and not strictly of gelatine), so that the fibres of wet 
hide are threads of colloid jelly. Water is also absorbed by the 
hide between the fibres by capillarity. 

It may be taken as fully proved that gelatine acts as a base 
with dilute acids, forming a colloid salt which ionises into a 
colloid kation and the anion of the acid, and which hydrolyses 
with excess of water back to gelatine and the original acid, so 
that the base can only be fully saturated in presence of excess of 
acid, and an equilibrium is formed dependent on the H+ con-, 
centration of the acid, which determines the proportion of un- 
combined gelatine and of free acid to that of saturated gelatine 
salt. As the latter rises gradually with the concentration of the 
acid, there is no point of neutrality to indicators, a fact which led 
earlier chemists to deny the existence of a definite compound. ^ 

^ T. Brailsford Robertson in his book on the Physical Chemistry of 
Colloids (Longmans, London and New York, 1918) admits the salt-forming 
character of proteias, but beUeves that it is not the terminal amino and 



COLLOID SOLUTIONS AND COLLOID STATE 117 

The point of saturation can, however, be determined by calculating 
the infinity-value of a plotted curve. 

If we place gelatine in dilute acid (say N/ioo HCl) it swells much 
more than in pure water, under favourable circumstances to fifty 
times its original weight, though as the acid concentration is 
increased beyond a certain optimum point, the swelHng steadily 
diminishes in a sort of hyperbolic curve, the greatest swelling 
being obtained with acid of H+ concentration about P^ =2-4. 
With pure water it only swells to seven or eight times its original 
weight. 

It must be understood that every mass of gelatine jelly or every 
gelatinous hide-fibre in an acid solution of any definite strength 
is in equilibrium with the surrounding solution, that is, that it 
contains water, gelatin-salt, free gelatin, and free acid in such 
proportions that there is no tendency for either acid or water 
from the surrounding solutions to pass into or out of the jelly, 
though both can pass freely through its surface. 

In applying these facts to the problem of swelling we may 
simplify the task by neglecting the un-ionised substances, which 
in this case passing freely in or out of the jelly, affect it only 
indirectly, and by confining our attention to the ions themselves 
which are proved to be the real active agents. 

Admitting the existence of gelatin salts, we have to consider 
the effect of their ionisation on the swelling. The gelatin-ion 
remains colloid, that is, it tends to agglomerate into masses or 
large particles which do not diffuse, and consequently exert no 
appreciable osmotic pressure, so that the ionised salt still remains 
a jelly or a colloidal solution. The anion, in this case the CI ion, 
on the other hand, tends to diffuse and exerts osmotic pressure, 
but cannot leave the jelly on account of the electro-chemical 
attraction of the gel-ion. It therefore swells the jelly, thus draw- 
ing into it the outside acid solution. This, however, contains the 
hydrogen and chlorine ions of the ionised acid in equal quantity, 
and while the former can enter the jelly without hindrance, the 
latter are opposed by the osmotic pressure of the ionised CI' 
already inside. The results are that the acid which enters is less 

carboxyl groups which, enter into combination, but the NH and CO groups 
at the Unkage of the different amino-acids which form the protein, and 
that consequently the colloid salts ionise into two colloid ions, and do not 
hydrolyse. It is possible that this may be true of some other proteins, 
or even of salts of gelatin with more concentrated acids, though it is not 
universally admitted, but it is certainly not the case with the monacid 
salts of gelatin with which we are concerned, as the chloride, for instance, 
certainly does ionise CI ions, and does hydrolj'se. This is also confirmed 
by the work of J. Loeb. 



ii8 PRINCIPLES OF LEATHER MANUFACTURE 

concentrated than that outside, that the total concentration of 
Cr is greater, and that of H"'" less in the jelly than in the outer 
solution, and as the acid H cannot enter without its associated 
CI, a layer of positive H+ forms outside the jelly surface, opposed 
and balanced by a similar layer of negative CI' within. Thus 
the two sides of the surface are in different electrical condition, 
or, in electrical language, there is a potential between them 
(Donnan's " Membrane Potential "),^ and the surface-layer 
outside the jelly has a small + electric charge. If, instead of 
being acid, the jelly and outer solution were alkaline, say with 
soda, the gelatin, being amphoteric, would form a sodium 
gelatinate in place of a gelatin chloride, and the charge would be 
negative instead of positive. It is obvious that between these 
conditions there must be an " isoelectric " point of neutrality at 
which there is no potential charge. This does, not necessarily 
cccur at the exact acid and alkaline neutrality of water, but is 
dependent on the relative acid and alkaline affinities of the 
individual proteid. In gelatin and hide-fibre it is slightly on 
the acid side of neutrality F^—4-'/, or N/5o,opo of actual hydrion- 
concentration, and this is the point of minimum swelling and 
greatest flaccidity of the skin, and the gelatin is insoluble and 
neutral, and on the acid side of the isoelectric point acts only as 
a base, and on the alkaline only as an acid. With regard to the 
actual tanning process, these charges are also of the highest 
importance, as negatively charged tannins can onlj^ combine with 
positively charged hide. 

The osmotic pressure tending to swell the jelly is therefore a 
balance of two opposing forces, that of H+ pressing in, and of 
cr (or some other acid-ion) pressing out. Donnan has shown 
that when the. jelly and its outer solution are in electrical and 
chemical equilibrium, the proportions of the two are connected 
by the law that the product H+ multiplied by the CI' within 
the jelly must equal that of the H+ x CI' of the outer acid. Now 
the H and CI of the outer acid are equal, while in the jelly CI is in 
excess, and the sum of equals is always less than that of unequals 
which give the same product. Thus the sum of 4+4 is 8, that of 
8+2 is 10, yet both give the product 16. Since the osmotic 
pressure is proportional to the sum of the ions, there is thus always 
a slight osmotic force tending to swell the jelly, greater as the two 
factors are more different. The greatest difference occurs when 
the quantity of free acid is very small and the chlorine in the jelly 
is almost entirely due to the ionisation of the gelatine salt, and it 

^ Donnan and Harris, J.C.S., 99, 191 1, 1554. Donnan, Zeits. f. Electro- 
chem., 17, 1911, 572. Abst. J.C.S., 100, 1911, 848. 



COLLOID SOLUTIONS AND COLLOID STATE 119 

is there that we get the greatest swelHng. As the concentration 
of the acid and its quantity in the jelly increases, the difference 
becomes less and the swelling diminishes, and if we add salt 
also in large excess, the chlorine-ion on both sides is enormously 
increased, and the difference will, as in pickling, almost entirely 
disappear. 

We have therefore accounted for the swelling force, and shown 
that as it increases or diminishes, the swelling does the same, and 
not merely in a general way, but mathematically and quantita- 
tively, and that this force only quite disappears when the con- 
centrations become infinite and equal. As, however, the swelling 
does not go on to infinity and solution, there must be some oppos- 
ing force which, when the swelling reaches a definite equilibrium, 
is equal and opposite to the swelling force. This is apparently 
the attraction of one gelatin particle for another, the elastic 
cohesion of the gelatin ; and it seems to follow Hooke's law 
generally applicable to elastic strains, in that it is proportional 
to the volume-extension. 1 The exact nature of such strains 
has not yet been determined, but in the case of colloids it may 
have something to do with the stretching or distortion of a 
molecular network. That it is sufficient to account even for the 
intense contraction under the influence of salt is shown by its 
magnitude when water is withdrawn by ordinary evaporation — 
a drying film of gelatin will often actually tear away the surface 
of glass to which it adheres. In another respect it resembles 
other elastic forces, since it diminishes rapidly and the swelling 
increases with increased temperature, till at the melting point 
of the jelly it apparently disappears, and the swelling goes on 
to complete or colloid solution. As, however, even in solution, 
the gelatin particles retain their electric charges, it is probable 
that, at least at temperatures below 70° C, they still continue as 
separate particles suspended in a surrounding liquid. 

The experimental work on which these results were based was 
of a very varied character, including determinations of hydrogen 
and chlorine ion concentrations by electrical methods with con- 
centration cells, and conductivities, but that principally used is 

1 It is interesting to note that this law would result if the increase of 
surface, and consequently of surface-energy, were proportionate to the 
increase of volume. It is not intended to suggest that the surface is actually 
increased, but merely that the elastic cohesion is due to the same forces 
of molecular attraction. Since the surface-energy bears a simple relation 
to the heat of volatilisation, the volume elasticity of any body obeying 
Hooke's law must do so also, and the idea suggests itself, that it should 
also be related to the heat of liquefaction of the jelly. 



120 PRINCIPLES OF LEATHER MANUFACTURE 

of so simple a character that it may be given in detail. To avoid 
complications caused by the liquid capillarily retained between 
the fibres of actual hide, thin sheets of the best French bone 
gelatine were mostly employed, but the same methods can be 
applied to skin itself with very approximate results, and as it is 
desirable that these should be extended, the writer would welcome 
any co-operative work of an accurate character. The earlier 
experiments were only intended as preliminary, and Coignet's 
gelatine was used without further purification, but in the later 
work it was carefully purified, so as to be almost ash-free, by 
slightly acidifying with SOg to remove calcium sulphite and 
phosphate, and then washing for some days on nets with many 
changes of distilled water. This gelatine, however, must have 
retained small quantities of alkali, since its ?„ in aqueous solution 
was 5-5, instead of 4*6 which corresponds to isoelectric gelatin. 
It was not found, however, that this had much influence on the 
determinations, since any traces of salts were pretty thoroughly 
removed by the considerable volumes of dilute acid employed in 
the experiments. As gelatin cannot be dried at a high tempera- 
ture without materially affecting its solubility, air-dry gelatin 
was preserved in a stoppered bottle, and moisture was deter- 
mined by drying a portion at 100° C. in vacuo, and this together 
with the traces of ash were allowed for in the weighings. Portions 
of I grm. were weighed into wide-mouthed stoppered bottles, 
and 100 c.c. of acid of accurately determined concentration 
was added, and allowed to stand with occasional gentle shaking 
for forty-eight hours, which was found to be sufficient to obtain 
practical equilibrium. The contents of the bottle were then 
poured into a funnel fitted with a finely perforated porcelain 
plate, covered with a clock-glass, and allowed to drain for two 
hours into a graduated cylinder. The volume subtracted from 
100 c.c. gave that absorbed by the gelatin, any traces of moisture 
which the air-dried gelatin contained being negligible against 
unavoidable losses by evaporation and liquid retained on the 
surfaces of vessels. The quantity was also determinable as a 
check by weighing the swollen jelly. The concentration of acid 
in the cylinder, with which the jelly was in equilibrium, was 
determined by titration with sodium hydrate and phenolphthalein, 
and that absorbed by the gelatin was calculated. We thus 
ascertain the total acid absorbed by the gelatin and the volume 
of the swelling produced. The absorbed acid is not, however, 
entirely combined chemically with the gelatin, but a considerable 
portion exists as free acid imbibed in the swelling, and the pro- 
portion of this to the acid actually combined was still to be deter- 



COLLOID SOLUTIONS AND COLLOID STATE 121 

mined. To accomplish this, advantage was taken of the fact 
that the addition of neutral salt, while it reduces the swelling, 
and expels the merely absorbed acid, does not affect that actually 
combined with the gelatin. The swollen jelly was returned to 
the wide-mouthed bottle, and common salt was added so long 
as it would dissolve. After standing at least twenty-four hours 
with repeated shaking equilibrium was again established, and the 
jelly was reduced to a horny mass retaining only about 1-5 c.c. 
of the solution. The acid present in the expelled salt solution 
was again determined by titration, and calculated to the whole 
volume of the solution originally absorbed, remembering that 
100 c.c. of a saturated salt solution only contain 88-6 c.c. of 
water. No appreciable error can arise from the assumption that 
the small remaining quantity of acid solution remaining in the 
jelly is of the same acid concentration as that expelled. ^ 

We have thus the means of determining (i) the free acid unab- 
sorbed which forms the " external solution " with which the 
jelly is in equilibrium ; (2) the free acid absorbed by the jelly ; 
and (3) the chlorine, ionised and non-ionised, combined with the 
jelly base. The sum of (2) and (3) can be further controlled by 
the titration of the dehydrated jelly with alkali hydroxide, which, 
with phenolphthalein as indicator, completely decomposes the 
gelatine salt. The table in App. B, p. 5gi, gives a series of 
such determinations with varying quantities of acid, and includes 
the whole of the results in the series of experiments to which 
they refer, which are more concordant than would be expected 
from the comparative roughness of the method. 

In the dilute solutions used the ionisation of the gelatin chloride 
was found to be complete, or at least equal to that of the hydro- 
chloric acid, which, to simplify calculations, was assumed to be 
wholly ionised, and we were thus enabled to give numerical 
values to the chlorion and hydrion concentrations, both of the 
jelly and of the external acid solution, and it was found that 
these, so obtained by calculation, agreed within experimental 
error with those obtained by the direct titration of the total 
hydrochloric acid in each (see p. 120). 

It is obvious that from the results of these experiments two 
distinct series of curves can be calculated : those of relative con- 

^ A saturated solution of salt at 15° C. contains 26-4 grm. of salt per 
100 grm., or 31-9 grm. per 100 c.c, and has a sp. gr. of 1-204. As, owing 
to the presence of HCl, the solution cannot be fully saturated with 
salt, it is probably sufficiently accurate for the purposes of this calculation 
to assume that 100 c.c. contains 88-6 c.c. of the original solution, or 100 
grm., 73-6 c.c. 



122 PRINCIPLES OF LEATHER MANUFACTURE 

centrations of the different constituents of the jelly and its 
equilibrium acid solution, which have been discussed, and a 
second series dealing with the actual quantities of each associated 
with I grm. or i mol. of gelatine, from which the composition of 
the gelatine salt and the combining equivalent of gelatine can 
be determined. When a strong acid combines with a strong 
base, the point of exact neutralisation is easily determined by 
titration with a suitable indicator; such as phenolphthalein ; but 
where either the acid or the base is very weak, the salt hj^drolyses 
again to free acid and base, and complete saturation of the one 
can only take place in presence of an excess of the other. Thus, 
in titrating gelatin with an acid, there is no point at which the 
indicator changes sharply, and the whole of the gelatin is only 
converted into chloride in presence of a large (and theoretically 
an infinite) excess of acid, a fact which led earlier chemists to 
suppose that no definite compound was formed. From the 
curves of concentration, however, we can determine the pro- 
portion of gelatin chloride formed as the acid is gradually in- 
creased in strength, and watch its rise, very rapid at first, but 
gradually becoming slower till it approaches a horizontal line at 
the point of complete saturation. To this point we cannot push 
it, since long before the acid has reached the requisite strength 
it begins to break up and dissolve the gelatin ; but if we can find 
a mathematical expression which exactly reproduces the part of 
the curve which we can observe, we can calculate the point at 
which the horizontal would be reached. As the case is one of 
ordinary hydrolysis, it is probable that it will be covered by 
Ostwald's hydrolysis formula, which for the purpose in view is 

X 

conveniently stated as , where x is the concentration of the 

•^ x + k 

external acid, and k an ordinary hydrolysis constant. In addition 
to k, we have also the molecular weight as a second " unknown," 
by which the weight of looo mgr. actually taken for the experi- 
mental curve must be divided to bring it to molecular proportions. 
Both these might be found by simultaneous equations from differ- 
ent points in the curve, or by the approximation method described 
in the original paper {Trans. Chem. Soc, 105, 1914, p. 319). The 

X 

actual curve was very accurately reproduced by 3'= • 

^ % + o-ooi3 

A X , which represents gelatine as a diacid base with 

a; + 1-05 839 

a second very weak affinity, and a molecular, or at least equiva- 



COLLOID SOLUTIONS AND COLLOID STATE 123 

lent, weight of 839. It seems probable, however, that gelatin is 
really monacid, and that the slight deviation of the curve was 
due to the splitting up of the gelatin by the more concentrated 
acid solutions ; and Wilson, adopting this view, calculates 
an equivalent weight of 768, and an empirical formula which 
agrees extremely well with the results of ultimate analysis. 
Collagen or hide-fibre is supposed to differ from gelatin by the 
loss of the elements of water, and if so, its equivalent weight 
must be 750. It is not to be supposed that this is the actual 
weight of the molecule in a gelatin solution, which is prob- 
ably often much larger, and probably variable by the union 
of several, or possibly many, equivalents (polymerisation). 
The equivalent represents the smallest proportion of gelatin 
which can combine with one equivalent of hydrochloric 
acid. 

The methods which have been described have proved quite 
satisfactory with " strong " acids, and as regards hydrochloric 
acid have been pretty thoroughly carried out ; but they fail to 
some extent with weak organic acids with which one can no 
longer assume complete ionisation of the salts, though that of 
the acids has been mostly determined. The addition of sodium 
acetate to gelatine acetate, for instance, fails to produce a dehy- 
dration nearly as complete as that of sodium chloride, so that the 
free acid in the jelly cannot be determined with much accuracy. 
The difficulty can be overcome with perhaps a slight loss of 
theoretical accuracy by the employment of sodium or potassium 
chloride as a dehydrating agent. This leads in the first instance to 
a quadruple equilibrium, in which the organic acid in the gelatin 
compound, from the large excess of the chloride, is practically 
entirely replaced by CI, and the gelatin chloride thus formed is 
dehydrated as before by the excess of the CI ion, the organic 
acid being titrated in the salt solution, and, if desired, the 
organic chloride estimated by gentle, ignition with known 
excess of sodium carbonate, and titrating back. This com- 
plete replacement of a weak acid by a strong one is at 
first sight somewhat surprising, but is in strict accordance 
with the mass law, and explains the possibility of effective 
pickling by weak organic acids. It is necessary to _^ add the 
salt to the original acid solution, as the use of an unacidified 
salt solution would cause extensive hydrolysis of the gelatin 
compound. 

The action of neutral salts on the gelatinous tissue demands 
further study. Loeb ^ seems to think that on the acid side of the 
^ Journ. Gen. Physiology, i, 39, etc., 1918. 



124 PRINCIPLES OF LEATHER MANUFACTURE 

isoelectric point the gelatin combines direct with their anions, 
and on the alkaline side with their kations ; but to the writer 
it seems more probable that what takes place is merely a double 
decomposition, and substitution of an element of the salt for one 
already in combination with the gelatin, such as has just been 
described. 



CHAPTER XI 

THE CHEMISTRY OF HIDE 

Chemical Constitution 

Introductory. — Apart from small quantities of inorganic matter 
and of pigment, the chemical constituents of hide belong to 
that class of nitrogenous organic compounds called proteins 
or proteids, compounds of quite exceptional interest, since they 
form part, often the major part, of all living structures, and are 
involved in all life processes. Many proteins are familiar to 
everybody. The white of eggs, for instance, consists of water and 
protein, largely egg-albumin, a protein easily coagulated by heat 
as is evident whenever an egg is boiled. Milk contains a coagul- 
able albumin, and casein, another protein. The former largely 
separates as a solid surface film when milk is heated, whilst 
casein is separated by the addition of small quantities of acid 
such as vinegar, or by a clotting ferment like that contained in 
rennet. All animal tissues are mainly protein. Skin, hair, 
nails and horns, muscle, etc., are composed of solid proteins, whilst 
blood contains, in addition to the suspended corpuscles and the 
fibrin, at least two other proteins in solution. In spite of the 
fact that chemists have been interested in proteins for over a 
century, we are still much in the dark in regard to their chemical 
constitution. A great deal has been accomplished, especially in 
the last twenty-five years, and we are now well acquainted with 
the simple chemical units of which proteins are composed. We 
know also the principal mode of linkage between these units. 
What we do not know is, first, the order or pattern in which the 
units are arranged — a matter of great consequence ; and second, 
the number of simple units needed to form the protein unit or 
molecule. The chemical investigation of proteins has been found 
very difficult, since many methods widely applicable in other fields 
of chemistry have proved almost useless. When it is realised 
that proteins crystallise only in a few instances, and then with 
difficulty, that they have no definite melting points or solubilities, 
and that they are all very similar in chemical composition, it 
becomes obvious that ordinary methods of purification, etc., are 
quite inadequate, and that a new technique is required. On this 

125 



126 PRINCIPLES OF LEATHER MANUFACTURE 

account progress was for a long time very slow, but the interest 
of the subject has attracted many workers in recent years, with 
the result stated above. 

Liebig's view was that one protein existed, and only one, and 
that the various substances which appeared to differ were simply 
more or less pure varieties of the one protein. It was by Liebig 
that a reliable method of ultimate organic analysis was elaborated, 
which when applied to proteins showed that a very great simi- 
larity in chemical composition existed amongst them. The 
amounts of carbon, nitrogen, and hydrogen do indeed fall within 
very narrow limits, as the following table of composition shows : — 

Carbon . . . 51-55 per cent. 

Hydrogen ... 7 

Nitrogen . . . 15-19 

Sulphur . . . 0-4-2 -5 ,, 

Proteins are known now whose composition falls outside the above 
figures, e.g. the protamines, but they are not commonly met with, 
and differ considerably in other respects from the general run of 
their class. It is, then, not surprising that Liebig thought of the 
proteins as one substance, difficult to obtain pure. Methods of 
arriving at chemical constitution were few in his day, "and 
empirical composition was almost the only sure and definite 
knowledge obtainable. Mulder held a modified view. He 
regarded protein as one parent substance which combined in 
various ways with other things, e.g. sulphur and phosphorus, to 
give the different substances occurring in Nature. This view, like 
that of Liebig, though reasonable enough in its day, has had to 
be abandoned by reason of the cumulative evidence of subsequent 
work. Very many different proteins are now believed to exist. 

The question may be asked. What are proteins ? Although 
the chemical (ultimate) composition of most of them is character- 
istic of their class, yet that particular composition is not invariable, 
nor confined to proteins. The same empirical formula ma}^ 
belong to substances showing the widest differences in nature 
and behaviour, and which could not reasonably be classed together. 
Proteins may be suitably described as nitrogenous organic sub- 
stances, always found in the colloid state, and which are broken 
up into amino-acids by boiling with acids or alkalies, or by the 
action of certain ferments or enzymes, e.g. trypsin. This breaking 
up is known as hydrolysis, and is of great importance. It forms 
part of the digestive processes, occurs in the liming and bating of 
hides, in the manufacture of glues and gelatines, and also in all 
putrefactions. On this account some explanation must be given, 
and first something must be said about acids and amino-acids. 



THE CHEMISTRY OF HIDE . 127 

Acids. — Almost all organic bodies possessing acidic properties 
{i.e. which can form salts containing metals) have in their mole- 
cules a hydroxy 1 ( — OH) group. The hydrogen of this group under 
appropriate circumstances can be replaced by a metal to form a 
salt. 

E.g. CgHgOH, phenol, and CgHjONa, sodium phenate. 
C2H5OH, alcohol, and CgHgONa, sodium ethylate. 

CH3 . CO . OH, acetic acid, and CH3 . CO . ONa, sodium acetate. 

Typical acids, in which acidic properties are well defined, have the 
hydroxyl group attached to a carbon atom, to which is also linked 
a second oxygen atom, the whole forming what is called the 
carboxyl group: — 

-Cf or -COOH. 

^OH 

In this way three of the four valencies of the carbon atom are 
involved, the fourth being used for a hydrogen atom or some 
monovalent group of atoms (monovalent radical). Examples 
of acids are : — 

1. H.COOH, formic acid. 

2. CH3 . CHg . COOH, propionic acid. 

3. CH3 . CH(OH) . COOH, hydroxy-propionic or lactic acid. 

4. CH3. CH(NH2)C00H, amino-propionic acid or alanine. 

It will be noticed that acids 2,3, and 4 are constitutionally closely 
related. Lactic acid differs from propionic in having a hydroxyl 
group attached to the middle carbon atom in place of one of the 
hydrogen atoms. In alanine, the amino-group (— NH2) plays 
the same part ; compounds similar to alanine are called aminO- 
acids, and form a numerous class, of which about eighteen can be 
found in the hydrolytic products of proteins. Indeed, these 
eighteen can be obtained from most single proteins, and the great 
majority in any case. The formulae of some of the simpler 
amino-acids are as follows : — 

Glycine or glycocoll, CH2(NH2)COOH, amino-acetic acid. 

Tyrosine, HO.C6H4.CH2.CH(NH2)COOH, hydroxyphenyl- 

amino-propionic acid. 
Glutamic acid, HOOC.CH2.CH2.CH(NH2)COOH, amino- 

glutaric acid. 

Lysine, CH2(NH2)CH2.CH2.CH2.CH(NH2)COOH, diamino- 
caproic acid. 



128 PRINCIPLES OF LEATHER MANUFACTURE 

In all amino-acids derived from proteins, one amino-group is 
attached to the carbon atom nearest to a carboxyl group. This 
applies to glutamic acid, which has two carboxyl groups, as well 
as to lysine, with two amino-groups. 

Amino-acids differ in one very important respect from other 
acids, the difference being due to the nature of the amino-group. 
This group is basic. As a consequence, amino-acids can form 
salts not only with bases but with acids. 

E.g. CH3 . CH(NH2)C00H -|- HCl-^CHg . CH . NH3CI . COOH. 
Cp. C6H5NH2 + HC1->C6H5NH3C1, aniline hydrochloride. 
H.NH2 + HC1->H.NH3C1, ammonium chloride. 

In the presence of bases, therefore, amino-acids act as acids and 
form metallic salts ; in the presence of acids, however, they act 
as bases and form hydrochlorides, etc. Tyrosine, for instance, is 
only soluble to a very slight extent in water, but dissolves readily 
in hydrochloric acid as hydrochloride and in alkali as tyrosinate. 
This behaviour is expressed in the word " amphoteric," a word 
implying the capacity of a substance to act as either acid or base 
according to circumstances. Proteins are also amphoteric by 
reason of the way in which they are built up of amino-acids. 
Gelatin, for instance, can form either hydrochloride or gelatinate. 
Hydrolysis. — The term hydrolysis used in our definition of pro- 
teins still requires explanation. From the point of view of organic 
chemistry, we may look upon hydrolysis as the breaking up of a 
compound by the addition of the elements of water. Experi- 
mentally this is only occasionally accomplished by simple treat- 
ment with water. In nearly every case it is necessary to heat the 
substance with more or less dilute acid or alkali. An example 
or two will help here. Take the case of ethyl acetate. If this 
substance be boiled with water, the reaction 

CH3 . COOCaHg-HHO . H-^CHg . COOH +C2H5OH 

will at first proceed in the direction indicated, but after a time 
an equilibrium is reached. If the complete hydrolysis of ethyl 
acetate is to be accomplished, acid or alkali must be used. Acid 
in sufficient strength, although it does not appear in the equation, 
will enable the reaction to be completed in a great number of 
cases, and all the acid originally used can be recovered at the end 
of the experiment. Alkali when used aids the hydrolysis by 
neutralising the acid product (forming sodium acetate in the 
example) until no more acid can be formed. Compounds which 
are hydrolysable in this way are those which we can imagine to 
have been formed by loss of water between two molecules. The 



THE CHEMISTRY OF HIDE 129 

formation of ethyl acetate from acetic acid and ethyl alcohol is 
such a case. Hydrolysis in the case of proteins is very often 
carried out by long boiling (six to twenty-four hours) with fairly 
strong acid (20 per cent. HCl or 30 per cent. H2SO4). More will 
be said on the experimental features in later sections of this 
chapter. For the present it must be understood that amino- 
acids form by far the greater part, often almost all, of the products 
of hydrolysis. It is therefore reasonable to assume that some 
mode of combination between amino-acids, a combination or 
" condensation " involving loss of water, is responsible for the 
typical protein molecule. 

Constitution. — ^It is generally believed that the union of amino- 
acids to form peptides is the principal mode of combination in 
protein formation. There are, indeed, several ways in which the 
amino-acids could combine with loss of water, but since Hofmeister 
in 1902 surveyed the evidence there has been little doubt that loss 
of water between the amino-group of one acid and the carboxyl 
group in the other is the principal mode of union in building up 
a protein. 



NH2 . R . CO |OH+H| . NH . RjCOOH 

-^NH^R . CO . NHR1COOH+H2O. 

The body represented on the right-hand side of the equation is 
called a dipeptide. Tripeptides, tetrapeptides, etc., contain the 
appropriate number of amino-acid radicals. For it will be 
realised at once that theoretically there is no limit to the number 
of amino-acids that can be combined together to give one poly- 
peptide. Experimentally the synthesis is difficult, but an 
oktadekapeptide containing eighteen radicals has been prepared, 
which proved to be remarkably like a natural protein. Assuming 
the polypeptide structure for proteins, hydrolysis is easily under- 
stood. Every —CO . NH— group is, by the addition of HO . H, 
converted into an acid (— COOH) and an amino-group (— NH2). 

NH2 . R . CO . NH . Ri . CO . NH . R2 . COOH-I-2H2O 

-^NHaRCOOH -f NH2R1COOH +NH2R2COOH. 

It is easy to see that the polypeptide structure gives the possi- 
bility of great variety and complexity in proteins. Plimmer 
states that there are 276 possible dipeptides. Possible tri- and 
tetrapeptides are far more numerous, as with increasing com- 
plexity the variations in arrangement of acid radicals which may 
occur increase out of all proportion to the number of radicals 
involved, according to the laws of permutations. If we consider 
three acid radicals, a, b, and c, and consider pentapeptides which 

9 



130 PRINCIPLES OF LEATHER MANUFACTURE 



they could form, we soon find that we could write down new ones 
apparently indefinitely : — 



a-a-a-a-a 



b-a-a-a-a 
a-h-a-a-a 
a-a-h-a-a 



c-a-a-a-a 
a-C'a-a-a 
a-a-c-a-a, and so on. 



Of course a polypeptide a-h-c is quite distinct from h-a-c. This 
complexity is made greater still by the formation of branched 
chains and ring structures, possibility of anhydride formation, etc., 
and the presence in many proteins of other groups not of a peptide 
character. Mucins, for example, contain carbohydrate. 



HOOC.CH.NH. 
I 

I 

CH, 

I 
CHo.CHNH, 



CO.CHR.NH. 



CO.CH.NH 

1 

CH, 



CO.CHR1.NH2 



CO.CHRg.NH.CO 



The example is a hypothetical pentapeptide having a ring 
structure. The broken lines show where hydrolysis will take 
place. When it is remembered that eighteen or more amino- 
acids may be involved in a protein molecule, it is easy to see that 
only by the merest chance can a natural protein be synthesised, 
even if we assume that we know all about the type of structure. 
As a matter of fact, this assumption would be far from justifiable, 
for in any case the analysis of a protein does not show the order 
in which the amino-acids have been linked. As it is here that 
the greatest causes of variety lie, it would appear that what is 
needed most is some method of experiment allowing amino- 
acids to be split off one by one or in small groups, so that the order 
obtaining in the protein can be worked out. 

It is possible for amino-acids to unite in other ways without 
loss of water between the molecules. As the amino-acids are 
both acids and bases, it is at least possible for them to combine 
with each other. 

NH2 . R . COOH +H2N . Ri . COOH-^NHaR . COOH3N . RjCOOH. 
Cp. CH3 . COOH-fH^N . H-^CHg . COOH3NH, 

ammonium acetate. 

And as polypeptides always contain both free amino-groups 



THE CHEMISTRY OF HIDE 131 

and free carboxyl groups, then it is possible for polypeptides to 
combine with each other in the same way, even if the amino-acids 
within them have combined by loss of water. Bayliss has 
shown that in the case of amino-acids at least this salt formation 
is more than a theoretical possibility. He obtained indubitable 
evidence from conductivity measurements that combination 
occurred when diamino-propionic acid and glutamic acid were 
brought together. Thus, though there is no direct evidence on 
the point, proteins may consist of polypeptides of moderate 
complexity combined with each other to form salts. 

Another possibility is that in proteins polypeptides are com- 
bined with each other through residual valencies in some such 
way as water of crystalhsation is held by many inorganic salts. 
Here again the only experimental evidence concerns amino-acids. 
Pfeiffer and Modelski have obtained with amino-acids and certain 
metalUc chlorides compounds of the type 

CaClalNHaRCOOH)^ . (Hp)^. 

Here the amino-acids as wholes combine with a metalHc salt as 
nucleus, and it is not difficult to imagine similar behaviour in the 
case of polypeptides. An interesting fact in this connection is 
that proteins are never quite free from small quantities of 
inorganic substances. 

Classification 

Some of the more important proteins are given in the following 
list :— 

Albumins. Egg-albumin, Serum albumin. — The chief char- 
acteristics of these proteins is that they are coagulated by heat. 
They are precipitated from aqueous solution by full saturation 
with ammonium sulphate. 

Globulins. — Ovoglobuhn from egg-white. Precipitated by half 
saturation with ammonium sulphate. Coagulable by heat, 
insoluble in water, but soluble in dilute salt solutions. 

Gliadins. — Wheat gliadin. Insoluble in water and absolute 
alcohol, but soluble in 50 to 70 per cent, alcohol. 

Scleroproteins. — Collagen, gelatin, keratins, elastin. These 
proteins do not much resemble one another. They are usually 
insoluble in water. They form the skeletal structures of animals. 

Phosphoproteins. — Casein. These contain phosphorus, prob- 
ably in combination with one of the amino-acids. 

Glucoproteins. — Mucins, in saliva and some tissues. These 
contain carbohydrate, which is split off on hydrolysis. 

Chromoproteins. — Haemoglobin, in blood. 



132 PRINCIPLES OF LEATHER MANUFACTURE 



Hydrolysis of Proteins 

The course of hydrolysis, whether effected by acids, alkalies, 

or enzymes, may be roughly represented by the following 

scheme: — 

Protein (collagen, casein, albumin, etc.) 

I 
Metaprotein (Lieberkiihn's jelly) 

Proteoses (gelatoses, albumoses) 

Peptones 

Simple Polypeptides (kyrins) 

Amino-acids. 

The protein is broken up into simpler bodies, and these again 
into others, until amino-acids are produced, which of course cannot 
be further hydrolysed. Thus metaprotein is only a little less 
complex than the original protein, and retains many of its pro- 
perties. Proteoses are formed by the hydrolysis of metaprotein, 
and so on. It must not be assumed that the whole of the original 
protein is converted into proteose before any peptone or amino- 
acid is formed. In some cases {e.g. hydrolysis of casein by 
trypsin) even amino-acid, the last product, is present from the 
very beginning of hydrolysis. Further, it must be understood 
that there is no sharp distinction between, say, albumoses and 
peptones, and that, on the other hand, different albumoses or 
different peptones may be present together amongst the pro- 
ducts of hydrolysis of a single protein. The usual tests and 
methods of separation for the products of h3^drolysis will now 
be given. 

Metaproteins. — ^These are only familiar in the hydrolysis of 
albumins and globulins, and are prepared by the action of dilute 
(0-4 to i-o per cent.) acids and alkalies at moderate temperatures. 
As they are insoluble in water they are precipitated on neutralisa- 
tion. Metaproteins are rendered insoluble in acid or alkali by 
boiling with water. They are distinguished from globulins by 
their insolubility in dilute solutions of neutral salts. 

Proteoses are soluble in water, but completely precipitated by 
saturation with ammonium sulphate. They are, in fact, divided 
into two classes, primary and secondary, the former being com- 
pletely precipitated by half saturation, the latter only by complete 



THE CHEMISTRY OF HIDE 133 

saturation with ammonium sulphate. There are also other 

distinctions. 

Primary Secondary 

Proteoses. Proteoses. 

Nitric acid . . . Ppt. No ppt. 
Potassium ferrocyanide 

and acetic acid . . Ppt. No ppt. 

Copper sulphate . . Ppt. No ppt. 

Although secondary proteoses appear to approach the peptones 
in properties, they are not as a rule regarded as formed from 
primary proteoses by further hydrolysis. Both kinds of proteose 
are formed at the same time, roughly speaking, from the protein 
molecule. 

Peptones are not precipitated by saturated ammonium sul- 
phate. They retain, however, sufficient of the protein character 
to give precipitates with gallotannin and phosphotungstic acid. 
Peptones are very soluble in water, and give evidence of lower 
molecular weight, as they diffuse, though only slowly, through 
parchment paper. An important reaction for the purification 
of peptones is due to Siegfried. After secondary proteoses have 
been renxoved by saturation with ammonium sulphate (pre- 
ferably in the presence of sulphuric acid), iron ammonium alum 
in saturated ammonium sulphate solution is added. Under 
these conditions peptones only, and no amino-acids, are pre- 
cipitated. Pure peptone can be recovered from this precipitate, 
and is colourless and ash-free, in contrast to the discouraging 
brown masses obtained by the earlier workers. Siegfried takes 
the view that peptones are not so simple in constitution (mol. 
wt. 400-600) as is often supposed, and regards the evidence from 
freezing-point determinations as unsound. Instead of peptones 
being fairly simple polypeptides corresponding to the above 
molecular weights, they are, he considers, feebly bound com- 
pounds of such polypeptides, largely dissociated in water, 
especially in' dilute solution. This would partly explain (i) the 
higher molecular weights found by freezing-point determinations 
when the peptone is dissolved in phenol, (2) the very great in- 
crease in molecular weight with increase in concentration. 
Siegfried has prepared bodies which he calls kyrins by further 
hydrolysis of peptones. For example, the peptone formed by the 
action of trypsin on gelatin was heated with 12-5 per cent, hydro- 
chloric acid at 38° C. until the specific rotation of the solution 
assumed a constant value. The kyrin in solution proved to 
be decidedly basic in properties, yielded arginine, lysine, and 



134 PRINCIPLES OF LEATHER MANUFACTURE 

glutamic acid in equimolecular proportions when fully hydro- 
lysed, and in fact was shown by further work to be a tripeptide. 

Polypeptides.— It is probably safe to assiune that some simple 
polypeptides are formed in the ordinary course of hydrolysis, and 
that the more complex peptones are not split up directly into 
amino-acids. The work of Siegfried on the kyrins supports this 
view. There is, however, an important point to be mentioned, 
namely, that acids and enzymes are to some extent able to form 
peptides and other anhydrides from amino-acids. In the course 
of protein hydrolysis, therefore, where the various stages are not 
shajply defined, it is. always possible that peptides present may 
have been formed from amino-acids resulting from the hydrolysis. 
The presence of peptides would then indicate a step back. 

Amino-acids have been previously described, and are the end 
products in complete hydrolysis. It has already been stated, 
however, that amino-acids may be present in certain cases from 
the very commencement of the process. 

General. — ^A large amount of the work done on products of 
partial hydrolysis has been with fibrin, albumin, or gelatin as 
original material, and it is doubtful whether the above classifica- 
tion could be easily or usefully applied in the case of every protein. 
In the case of keratin, for instance, hydrolysis has to be carried 
further than with gelatin before soluble products are obtained. 
Gelatin dissolves in cold lime water in a day or two, and the solu- 
tion gives no precipitate on neutralisation. Keratin is acted upon 
much more slowly, and the solution obtained {e.g. a used lime 
liquor) invariably gives a precipitate when neutralised. 
Obviously in the latter case it is impossible to test the behaviour 
with ammonium sulphate solutions. In this connection it is 
interesting to notice the complexity of used tannery lime liquors. 
We have present proteoses, peptones, amino-acids, etc., derived 
from at least four different proteins. These substances are in 
the form of calcimn salts. Further, since certain bacteria present 
have the power of acting upon and " deaminising " amino-acids, 
we have in lime liquors free ammonia, and also acids (as calcium 
salts) derived from amino-acids by loss of ammonia. 

Acid Hydrolysis. — Protein is heated with five or six times its 
weight of concentrated hydrochloric acid or 25 to 30 per cent, 
sulphuric acid. Until the protein is quite dissolved, the heating 
should be carried out on the water-bath. The solution is after- 
wards boiled under a reflux condenser until hydrotysis is com- 
plete, i.e. until the biuret test (p. 140) fails. This often takes 
about six to twelve hours with hydrochloric and twelve to 
twenty-four hours with sulphuric acid, but may require two or 



THE CHEMISTRY OF HIDE 



135 



three days. After hydrolysis, most of the acid is removed by 
distillation in vacuo (or by addition of baryta if sulphuric acid 
has been used), and a slightly acid liquor obtained containing 
some quantity of black insoluble matter or humin. After 
filtration and decolorisation the solution is exactly neutralised 
and evaporated to a small bulk. Tyrosine and cystine crystallise 
out. The diamino-acids (arginine, lysine, and histidine) are then 
precipitated by phosphotungstic acid, and can be separated from 
the precipitate. The filtrate contains monoamino-acids, which 
are converted into their ethyl esters, and separated by fractional 
distillation of the esters at very low pressures (0-5 to 10 mm.). 
This is the method elaborated by E. Fischer, which was the first 
step towards the complete analysis of proteins. Large numbers 
of quantitative hydrolyses have been carried out, and a few of the 
results are given below. The figures are from tables in Plimmer's 
book. 

Keratin Keratin Albumin 

Gelatin. Elastin. from from Casein. from 

Horse hair. Wool. Blood. 

Per cent. Per cent. Per cent. Per cent. Per cent. Per cent. 



Glycine . 

Alanine . 

Valine 

Leucine . 

Isoleucine 

Phenylalanine 

Tyrosine 

Serine 

Cystine . 

Proline . 

Oxyproline 

Aspartic acid 

Glutamic acid 

Tryptophane 

Arginine 

Lysine 

Histidine 

Ammonia 

Total 



i6-5 

0-8 
i-o 

2-1 


0-4 
o 

0-4 
o 

77 

3-0 

0-6 

0-9 

o 

7-6 

2-8 

0-4 
0-4 



25-8 
6-6 
i-o 

21-4 

3-9 

0-4 



17 

+ 
0-8 

0-3 



47 
1-5 
0-9 
7-1 

0- 

3-2 

0-6 
8-0 
3-4 

0-3 
37 

4-5 

i-i 
0-6 



0-6 

4-4 

2-8 

II-5 



2-9 
o-i 

7-3 
4.4 

2-3 
12-9 



4^-6 6i-9 39-6 



49-2 



o 

0-9 

10 

10-5 

3-2 

4-5 

0-3" 

o-i 

3-1 

0-3 

1-2 
II-O 

1-5 

4-8 

5-8 
2-6 
1-6 



51-4 42-8 



o 
27 

20-0 

31 

2-1 

0-6 

2-5 
I-o 

3-1 
77 

+ 



It will be noticed that in the above examples only about half 
the protein is accounted for. This is due in some cases to the 
fact that certain acids have not been estimated, but much more 
to the inevitable losses in the determination of monoamino-acids 
by Fischer's ester method. Recently Dakin has been able to 



136 PRINCIPLES OF LEATHER MANUFACTURE 

make very great improvements in the estimation and separation 
of monoamino-acids. He extracts the aqueous solution with a 
non-miscible solvent, butyl alcohol, in which indeed the amino- 
acids are only very slightly soluble. But a continuous extraction 
process is used, similar to the extraction of tannin solutions by 
amyl acetate in qualitative analysis. Some of the amino-acids 
crystallise out from the alcohol during the extraction, others are 
not extracted. This method has been applied to the quantitative 
hydrolysis of gelatin, with the very interesting result that over 
91 per cent, of the protein is now accounted for. Dakin's figures 
are given below, and should be compared with those of the older 
analysis (p. 135) which stood for many years as the best 
available : — 

Per Per Per 

cent. cent. ' cent. 

Glycine . . 25-5 Proline . , 9-5 Arginine . 8-2 

Alanine : . 8-7 Oxyproline , 14-1 Lysine . 5-9 

Leucine . 7-1 Aspartic acid 3-4 Ammonia . 0-4 

Serine . .0-4 Glutamic acid 5-8 

Phenylalanine 1-4 Histidine . 0-9 Total . 91-3 
Tyrosine . O'oi 

Alkaline Hydrolysis. — This is of very great interest in leather^ 
manufacture, but has received much less attention than acid 
hydrolysis. Results so far obtained go to show that the products 
by the two methods do not differ very much. Alkali seems to have 
the more powerful action. It has been stated that gelatin soon 
dissolves in cold lime water. In N/20 acid, however, hydrolysis 
is very slow at ordinary temperatures. In some cases resistant 
bodies formed in acid hydrolysis are much more quickly broken 
up by alkali. Abderhalden found that a resistant body obtained 
from silk could only be hydrolysed further by the use of alkali. 
Two other important points are: (i) cystine and arginine are 
decomposed by alkali, and (2) optically active amino-acids are 
rendered completely inactive. 

A study of slow alkali hydrolysis at ordinary temperatures 
would be of very great interest to leather trades' chemists, 
especially with regard to the nature of the liming process. The 
great difficulties presented have no doubt deterred workers from 
taking up this subject. 

Enzyme Hydrolysis. — This subject is of paramount importance 
to physiologists and biochemists, and has been the subject of much 
investigation. In leather manufacture enzyme hydrolysis occurs 
in both liming and bating, and must therefore receive some con- 



THE CHEMISTRY OF HIDE 137 

sideration. Several enzymes are capable of breaking up proteins, 
and amongst the most important are pepsin and trypsin. Such 
hydrolysis, however, is never complete. A resistant body named 
antipeptone is formed, which is never completely broken up 
even by the combined, or rather successive, action of different 
enzymes, though almost complete hydrolysis can be obtained in 
some cases if a sufficiently long time is allowed. Fischer and 
Abderhalden found that antipeptone contains all the phenyl- 
alanine and proline of the original protein. Of the two enzymes 
mentioned, trypsin effects the more complete hydrolysis, pro- 
ducing as a rule considerable quantities of amino-acids, whereas 
pepsin rarely carries hydrolysis past the peptone stage. The 
course of action of pepsin on fibrin is according to Cole as 
follows : — 

Fibrin 

Soluble globulin 

I 
Metaprotein 



Primary albumoses Secondary albumoses 



Peptones. 
Kiihne's scheme for tryptic digestion is — 
Egg-albumin 
Secondary albumoses 



Antipeptone Hemipeptone 

Amino-acids. 

With regard to this latter scheme, it must be stated that the 
hemipeptone is a hypothetical substance and has never been 
isolated, and that some observers have found that nearly all 
the tyrosine is separated from casein in the first hour of trjq^tic 
digestion. 

Enzyme hydrolysis is much affected by conditions of tempera- 
ture, reaction, dilution, etc. Pepsin requires an acid medium, 
and is most effective in 0-2 per cent, hydrochloric acid, or at 



138 PRINCIPLES OF LEATHER MANUFACTURE 

Ph=i-4 about. Weaker acids must be used in appropriately 
higher concentrations. In alkaline solutions pepsin is very 
rapidly destroyed, especially on warming. It has been stated 
that peptic digestion produces nothing simpler than peptones and 
polypeptides. Indeed it would appear, according to Cole, that 
pepsin cannot break any peptide linkage since it does not hydro- 
lyse any synthetic polypeptide. Trypsin acts best in alkaline 
solution at Ph about 8-i. It is not, however, destroyed by 
acids, but, according to some observers, is, in the absence of 
protein, even more stable in acid than in alkaH. In carrying 
out hydrolyses by means of trypsin, care should be taken to 
have the reaction mixture alkaline to cresol red and acid to 
phenolphthalein. Trypsin acts on all soluble proteins, but not 
on all insoluble ones {e.g. collagen). It is sometimes necessary 
to give insoluble proteins a preliminary treatment with dilute 
acid or alkali or with pepsin. Hydrolysis will then in any 
case be more complete. It would appear that certain linkages 
in the protein molecule are opened up by pepsin, but not by 
trypsin. The products of tryptic digestion are simple poly- 
peptides and amino-acids, the hydrolysis never being complete 
even after prolonged action. Trypsin is of great interest in 
leather manufacture, owing to its capacity for unhairing and 
bating skins. Probably the unhairing action depends on the 
digestion of the softer keratin of the epidermis, since the outer 
horny layer can often be removed bodily with the hair. The use 
of trypsin in bating wiU be discussed in a later chapter. All that 
need be said here is, that it appears to digest rapidly the elastin 
of the elastic fibres whilst having only a very slow action upon 
the collagen fibre 

General Characters of the Proteins 

Solubility, etc. — Some important proteins belonging to the 
class of scieroproteins appear to be quite insoluble in the un- 
changed condition, notably collagen, elastin, and the keratins. 
Acids and alkalies effect solution especially with the aid of heat, 
but hydrolysis has first taken place. Gelatin is insoluble in cold 
water, but always swells to several times its original volume. 
The swollen jelly on warming easily melts (at about 25° C), 
giving a solution of gelatin. Very concentrated solutions can be 
prepared with hot water, but it should here be remarked that in 
no case should a gelatin solution be heated above 70° C. unless 
the hydrolysis which then begins is of no moment. Casein is 
insoluble in water, although particles suspended in water are able 



THE CHEMISTRY OF HIDE 139 

to redden blue litmus paper. It dissolves in alkali to form 
caseinate, and also in acid. Albumin swells somewhat, and dis- 
solves in cold water. The mucins dissolve easily in dilute alkalies, 
and may be precipitated by neutralisation with acetic acid, 
though they are soluble in weak (o-i per cent.) hydrochloric acid. 
Products of hydrolysis are in general soluble in water. Even 
insoluble proteins soon yield soluble products when hydrolysed. 
There are, however, exceptions, some of the amino-acids, for 
instance, not being very soluble. 

Alcohol and organic liquids generally do not dissolve proteins. 
Gelatin, for instance, is readily thrown out of aqueous solution 
by addition of alcohol. On the other hand, some peptones and 
proteoses are soluble in alcohol. The gliadins behave remarkably 
in this respect. Wheat gliadin is perfectly insoluble in either 
water or pure alcohol, but dissolves in 70 to 80 per cent, alcohol. 
Urea solutions dissolve most proteins, probably by salt formation, 
since urea is a base. In a saturated solution of urea, gelatin 
will dissolve to the extent of 40 per cent, at room temperature. 
If the urea is removed by dialysis a jeUy results. 

Not much is known with regard to the degree of solubility of 
proteins in solutions of salts. Very considerable differences exist 
however, and afford means for the characterisation and separation 
of proteins. Globulins, for example, are insoluble in water, but 
soluble in dilute (10 per cent.) solutions of common salt. Higher 
concentrations cause precipitation (coagulation, " salting out "). 
Serum albumin begins to be precipitated from i per cent, solution 
by addition of 33 per cent, of ammonium sulphate, and is com- 
pletely thrown out by 47 per cent, of the salt. Other salts have 
usually less precipitating power than ammonium sulphate, and 
are not so useful. Saturated magnesium sulphate is usually 
regarded as equivalent to half-saturated ammonium sulphate. 
For the details of the fractional salting out of proteins the reader 
is referred to the text-books. 

Precipitation by Reagents. — Another mode of precipitation, in 
which the reagent chemically combines or reacts with the protein, 
is effected by various substances, notably the reagents used for 
precipitating alkaloids and the salts of heavy metals. The more 
important are given below. 

(i) Mineral acids, e.g. nitric and metaphosphoric acids. 
HeUer's test is performed by adding nitric acid slowly to the 
protein solution. A white ring appears at the surface of separa- 
tion of the two fluids. 

(2) The alkaloidal reagents include phosphotungstic, phospho- 
molybdic, ferrocyanic, picric and sulphosalicylic acids, and 



140 PRINCIPLES OF LEATHER MANUFACTURE 

tannin. The protein solution should be slightly acid before the 
reagent is added. 

(3) The salts of heavy metals most often used are lead acetate, 
zinc and copper sulphates, and ferric chloride. The protein 
solution should be faintly alkaline. The precipitates formed are 
often soluble in acid or in excess of reagent. 

The reactions under (2) require the protein to be in the acid or 
positively charged condition, as the reagents have a complex 
negative ion which is believed to be the effective agent in the 
reaction. The behaviour of tannin is explicable on similar 
grounds, as it is a negatively charged colloid in acid solution. In 
the reactions under (3) the protein is negative and the positive 
kation of the reagent is effective. A consideration of these 
reactions shows that substances such as copper sulphate and 
mercuric chloride can only be used under special conditions for 
the sterilisation of hides, since chemical combination may occur 
and unfavourably influence the subsequent processes of leather 
manufacture. 

Colour Reactions. — Xanthoproteic Reaction. — This is carried 
out by the addition of strong nitric acid to the protein or its 
solution and then boiling. A yellow precipitate or solution is 
obtained, which becomes orange on the addition of alkali. 

Millon's Reaction. — Millon's reagent is prepared by dissolving 
one part by weight of mercury in two parts of nitric acid (sp. gr. 
1-42), warming if necessary towards the end of the reaction. 
The solution is then diluted with two volumes of water, allowed 
to settle, and filtered. When the reagent is added to a protein 
solution a whitish precipitate is formed, which becomes brick red 
on careful warming, or else dissolves, giving a red solution. This 
test only gives a positive result with proteins containing tyrosine. 
The xanthoproteic reaction is positive with proteins containing 
t3n'osine, tryptophane or phenylalanine, i.e. the aromatic amino- 
acids. 

Biuret Test. — According to Cole, 3 c.c. of protein solution (about 
I to 2 per cent.) is heated with i c.c. of 40 per cent, sodium 
hydroxide and one drop of i per cent, copper sulphate solution. 
A pink or violet colour is given not only with proteins but with 
proteoses and peptones. 

Glyoxylic Reaction (Hopkins and Cole). — Protein solution is 
mixed with glyoxylic reagent, and an equal volume of concen- 
trated sulphuric acid is poured down the side of the tube. A 
purple colour is seen at the surface of separation of the fluids, 
and with gentle shaking this colour spreads throughout the 
contents of the tube. The glyoxylic reagent is prepared by 



THE CHEMISTRY OF HIDE 141 

reducing oxalic acid with sodium amalgam or magnesium powder. 
The reaction is shown only by those proteins which contain the 
amino-acid tryptophane. Indeed the discovery of this substance 
by Hopkins and Cole was due to the study of the glyoxylic test. 

Sulphur Reaction. — Protein is boiled with 40 per cent, caustic 
soda for a minute or two, and then a little lead acetate is added. 
A blackening results by the formation of lead sulphide from the 
sulphur of the protein. 

Molisch's Reaction. — Protein solution is treated with a few 
drops of I per cent, solution of a-naphthol or of thymol in alcohol. 
After mixing, concentrated sulphuric acid is poured down the 
side of the test-tube. A violet colour appears at the surface of 
separation. This reaction is positive with proteins [e.g. mucins) 
which contain carbohydrate. 

Crystallisation of Proteins. — Although crystalline bodies of a 
protein nature have been known for many years, it is only 
recently that the crystahisation of proteins has become at all 
generally practised. Aleurone grains were first noticed in 1850 
in gluten, and occur in the seeds of many plants {e.g. hemp seed). 
Although they look like crystals, yet they behave differently from 
ordinary crystals. They swell, for instance, in water. In recent 
years, however, many proteins have been artificially crystallised, 
such as edestin, egg-albumin, gelatin, and haemoglobin. The 
typical method is that of Hopkins and Pinkus for the crystallisa- 
tion of egg-albumin. A solution of this protein half saturated 
with ammonium sulphate is prepared, and cautiously treated 
with 10 per cent, acetic acid until a permanent precipitate 
appears. A specified additional amount of acid is then added, 
which produces a heavier precipitate. The mixture is allowed 
to stand overnight, when the precipitate becomes crystalline. 
The protein can be recrystallised as often as desired. Other 
proteins are sometimes crystallised by means of alcohol. Von 
Weimarn, for instance, adds alcohol to an aqueous solution of 
gelatin until a slight permanent precipitate is formed. The 
mixture is then left in a desiccator containing strong sulphuric 
acid, by which means water is more rapidly removed than the 
alcohol. Crystallisation begins as the solution becomes richer in 
alcohol, and the insolubility of the gelatin increases. On the 
whole, however, it cannot be claimed that the crystallisation of 
proteins has at present any outstanding importance. Crystals 
obtained from ammonium sulphate solutions are always largely 
contaminated with the salt, and must be further purified by 
solution and dialysis. In any case, there is some doubt as to the 
identity of crystallised products and the original proteins. It 



142 PRINCIPLES OF LEATHER MANUFACTURE 

should be mentioned that as a rule the crystals obtained are only 
recognisable as such under the microscope. 

Coagulation by Heat. — Some proteins, i.e. albumins and 
globulins, are coagulated by heat. Egg-albumin coagulates 
when its solution is heated to about 55 to 60° C. This is of course 
a familiar occurrence seen every time an egg is cooked, but the 
nature of the change involved is quite obscure. There are, how- 
ever, several points worthy of notice. In the first place, the 
coagulation differs from that produced by ammonium sulphate, 
etc., in being irreversible. Heat-coagulated egg-white cannot be 
brought back into solution. In the second place, the presence 
of alkali, acid, or of neutral salts (even in small quantities) is of 
great influence. Dialysed solutions often lose their coagulability 
owing to the presence of small quantities of alkali or acid, but 
long-continued dialysis wiU effect a recovery 'of this property. 
In the case of a dialysed solution no longer coagulable small 
quantities of neutral salt will, like further dialysis, render 
coagulation possible. The amounts required are much smaller 
in the case of calcium and magnesium salts than with alkali 
salts. Increase in the amount of salt raises at first the coagula- 
tion temperature sometimes by as much as 15 to 18° C, but may 
later cause a fall. It is therefore obvious that the coagulation 
temperature can only be characteristic of a protein under exactly 
defined circumstances. A non-coagulable protein may be freed, 
say, from albumin by heating the solution and filtering, but, as 
indicated above, such a process does not produce pure albumin. 

Phosphorus and Sulphur Content. — Almost all proteins contain 
sulphur. The amount present is usually quite small, and varies 
very much in different proteins. Peptones are, as a rule, 
free from sulphur. One class of proteins, the keratins, is dis- 
tinguished by comparatively high sulphur content (2 to 5 per 
cent.), few others containing so much as 2 per cent. Only one 
amino-acid, cystine, contains sulphur, and the amount of this 
element is often taken as a measure of the cystine content of a 
protein. This procedure is perhaps not free from objection. 

A few proteins contain considerable amounts of phosphorus. 
Of these the best known is casein, which contains 0-85 per cent. 

Distribution of Nitrogen in Proteins. — It has already been 
pointed out that the amount of nitrogen contained in a protein 
rarely varies very much. Nevertheless the distribution of 
.the nitrogen, i.e. the amounts combined in certain groups 
of amino-acids, shows considerable variation, and is often 
useful in characterising proteins. A rapid method for the 
determination of the distribution was first worked out by 



THE CHEMISTRY OF HIDE - 143 

Hausmann, and the figures obtained are often referred to as 
' ' Hausmann numbers . " The original procedure has been modified 
by several workers, notably Osborne, whose method is in outline 
as follows. One grm. of protein is completely hydrolysed, freed 
from most of the acid used, and subsequently distilled with 
magnesia. The ammonia liberated is equivalent to the " amide " 
nitrogen, which is believed to exist in the protein in — CONH2 
groups. The remaining solution is filtered. The residue is not 
only magnesia, but contains the black precipitate formed in 
hydrolysis. The whole residue after washing is analysed by 
Kjeldahl's method, and the result gives the " humin " nitrogen. 
The filtrate contains the monamino- and diamino-acids. The 
latter acids are with proper care precipitated by phosphotungstic 
acid, and the " diamino " nitrogen determined by analysis of the 
precipitate. The " monamino " nitrogen is calculated by sub- 
tracting the sum of the three quantities already determined from 
the total nitrogen of the original protein. In illustration of the 
results obtained, zein obtained from maize and conalbumin from 
egg-white may be compared. Each protein contains 16 per cent, 
of nitrogen. Of this amount, 18-4 per cent, in the case of zein 
and 7-51 per cent, in the case of conalbumin is amide nitrogen, 
77-6 per cent, and 65-1 per cent, monamino nitrogen, 3-0 per 
cent, and 25-8 per cent, diamino nitrogen, and i-o per cent, and 
17 per cent, humin nitrogen. This example clearly shows the 
usefulness of the method. A further separation of nitrogen into 
seven groups is often carried out. The method, however, is too 
long to be described here. 

Action of Nitrous Acid on Proteins and Determination of free 
Amino-groups. — The well-known reaction between amino-groups 
and nitrous acid takes place, as would be expected, with proteins, 
since the presence of free amino-groups is a priori very probable. 
There is usually decided action accompanied by frothing and 
evolution of nitrogen. Amongst the most important of the early 
work was that carried out by Skraup, who prepared what he 
termed " desamido-proteins," and then investigated the products 
of hydrolysis of these bodies. The important conclusion to be 
drawn from this work is, that the diamino-acid lysine whilst 
combined in the protein molecule is acted upon and changed. 
All the other amino-acids were found in the hydrolysis products 
of the desamido-protein just as in the case of the original protein. 
Lysine, however, was never found. A second important fact 
due to Levites is that the amount of amide nitrogen (see above) in 
a protein is unaltered by the action of nitrous acid. The reaction 
was first studied quantitatively by Sachsse and Kormann in 



144 PRINCIPLES OF LEATHER MANUFACTURE 

1875, and much work has been done since by Brown and MUlar, 
van Slyke, Fischer and Koelker, and others, and the estimation 
of nitrogen hberated by the action of nitrous acid on proteins 
is now a regular laboratory process described in text-books of 
physiological chemistry, etc. With a pure amino-acid the reaction 
is usually simple, as in the case of aspartic acid : 

HOOC . CH2 . CH(NH2)C00H +H0 . NO 

-^HOOC . CH2 . CH(OH) . COOH+N2 + H2O. 

Half the nitrogen evolved comes from the amino-acid. Anomalies 
occur, however, even with simple acids. Both glycine and cystine 
give a larger volume of gas than the simple equation accounts for. 
Fischer and Koelker found the reaction between polypeptides and 
nitrous acid to be somewhat indefinite and irregular, and that the 
nitrous acid reacts to some extent with the peptide .( — CO . NH— ) 
linkages. Nevertheless the interesting conclusion has been drawn, 
that in the case of proteins the nitrogen formed with nitrous acid 
comes entirely from one of the two amino-groups of lysine. A 
protein containing no lysine should therefore yield no nitrogen. 
This has been shown to be true in the case of zein, a protein 
obtained from maize. With o^her proteins it has been shown 
that the nitrogen obtained corresponds to half the lysine in the 
molecule. This work has received much support from the 
formaldehyde reaction which is described below. It is clear that 
during the hydrolysis of a protein the number of free amino- 
groups increases, and that an increasing proportion of the total 
nitrogen becomes reactive with nitrous acid. Edestin, for 
instance, contains i-8 per cent, of its total nitrogen in free 
amino-groups, but when fully hydrolysed no less than 79 per cent, 
of the nitrogen is in that form. Clearly the nitrous-acid reaction 
gives valuable information as to the progress of hydrolysis. 

Action of Formaldehyde on Amino-acids and Proteins. — Amino- 
acids are very weak acids, and it is not easy to titrate them with 
alkali in the ordinary way, owing to the hydrolysis of their 
sodium salts. If, however, they are allowed to react with 
formaldehyde, the basic character of the amino-group is destroyed, 
and very much stronger acids are produced, which can be easily 

.NH2 ,C.N:CH2 

R/ +H.CHO->R<: +H2O 

COOH \COOH 

titrated with alkali. The formaldehyde used should be neutral 
to phenolphthalein, the indicator used. The acidity developed 
by the addition of formaldehyde is thus a measure of free amino 



THE CHEMISTRY OF HIDE 145 

and carboxyl groups, though in most cases no exact calculation 
can be made, since the various amino-acids are present in unknown 
proportions. Clearly, during the course of hydrolysis of a protein 
the formaldehyde titration will very markedly increase, just as 
the volume of nitrogen liberated by nitrous acid increases ; we 
should therefore have a means of following the course of hydrolysis 
and verifying the information given by the van Slyke nitrous- 
acid method. In practice the two methods do as a rule give 
parallel results, but there are one or two points about the formal- 
dehyde titration which are causing it to lose favour. In the first 
place, formaldehyde does not react with some anhydrides of amino- 
acids. These bodies are certainly formed in hydrolysis, especially 
when prolonged, as has been shown by Fischer and Abderhalden. 
On this account alone, therefore, the formaldehyde titration will 
fail to give a correct indication of the course of hydrolysis. If 
amino-acids were being produced just as quickly as amino-acids 
already formed were condensing into anhydrides, then the titra- 
tion would show no change. A second objection is that the 
reaction between histidine (an amino-acid) and formaldehyde is 
abnormal and irregular. Finally, the reaction in the case of any 
amino-acid is reversible, and hence in the titration an excess of 
alkali must be used, e.g. by titrating to a decided red colour with 
phenolphthalein. In spite of all this, however, it must be con- 
ceded that the method is certainly useful, one great advantage 
lying in the ease with which it can be carried out. 

The action of formaldehyde on hide appears to be analogous 
to the action on amino-acids. A certain amount of acidity is 
always developed, due no doubt to the change in the amino- 
groups described above. The reversibility of the action on amino- 
acids, except in the presence of excess of alkali, is indicated in 
the case of proteins by two facts : (i) the formaldehyde can be 
easily split off quantitatively by distilling the compound in 
steam or by boiling with dilute (N/io) acid ; and (2) formaldehyde 
tannage is usually carried out in the presence of alkali. Qualita- 
tively, proteins show considerable changes on treatment with 
formaldehyde. They are rendered resistant to pepsin, and prob- 
ably to other enzymes, though a digestible protein is easily 
recovered by treatment with steam. Egg-white appears to lose 
its property of coagulating on heating. Soluble proteins often 
become insoluble. Gelatin, for instance, becomes a hard in- 
soluble substance, and the setting point of gelatin jellies rises. 
If very dilute (|- per cent.) formaldehyde solutions are used, the 
reaction appears to require two or three weeks for completion. 
The time is much shorter when strong solutions are used. The 

10 



146 PRINCIPLES OF LEATHER MANUFACTURE 

amount of formaldehyde which combines with a given weight of 
protein has been investigated by Benedicenti, who added 4 ex. 
of 2 per cent, formaldehyde solution to 10 c.c. of protein solution. 
His results, given by Schryver, are as follows : — 

I grm. gelatin combines with 0-0135 grm, formaldehyde. 

10 c.c. fresh egg-white ,, „ 0-375 



2 grm. powdered 
10 c.c. blood serum 

3 grm, fibrin 
5 grm. casein 



0-036 
0-315 
0-0345 
0-0294 



These quantities will probably be onl}^ correct under certain 
experimental conditions, and too much importance should not be 
attached to them. 

Action of Halogens, etc., on Proteins. — The action of halogens 
on proteins is complex, involving both oxidation and sub- 
stitution. In spite of the very considerable amount of work 
which has been done on the subject, we have little or no clear 
understanding of the reactions involved. All proteins react 
directly with halogens, sometimes forming derivatives containing 
as much as 15 per cent, of halogen, as in some bromine 
compounds of egg-albumin, serum-globulin, proto- and deutero- 
albumose. These derivatives are readily soluble in alkaline 
solutions, from which they can be reprecipitated by acids. Some 
bromine derivatives are soluble in alcohol but not in the fat 
solvents. They can be salted out from alkaline solution by means 
of ammonium sulphate, but are not precipitated by the alkaloidal 
reagents ; they give the biuret reaction, but not the Millon and 
glyoxylic tests. At the same time as the halogen derivatives are 
formed, certain groups appear to be eliminated from the protein 
molecule. In the reaction products with iodine, ammonium 
iodide and iodate, iodoform, carbonic, formic, and acetic acids 
have been found, and it is supposed that these substances 
result from the scission of particular groups. 

Collagen 

Hide or skin consists of several proteins, namely, collagen 
forming the white fibres of the corium, keratins forming- the 
epidermis, and hair or wool, mucins and albumins in small quan- 
tities in the corium, partly arising from blood and lymph, and 
also the elastin of the yeUow elastic fibres. In the fresh hide 
collagen occurs in the swollen, hydrated condition, and although 
it is the only important hide constituent, very little is known of its 



THE CHEMISTRY OF HIDE 147 

chemical nature beyond what is deduced from the chemistry of 
the closely allied substance gelatin, which is described below. It 
is probable that there are several collagens derived from different 
hides and skins. The protein constituent of bone, which like 
hide yields gelatin on boiling with water, is also probably another 
collagen. There is, however, no real proof available, though 
analyses of purified corium from various sources show differences 
and support the above view. Von Schroeder and Paessler 
obtained the results given below : — 

Source. C. H. N. O. 

Ox, calf, horse, pig, 

camel, rhinoceros . 50-2 6-4 17-8 25-4 

Goat and deer . . 50-3 6-4 17-4 25-9 

Sheep and dog . . 50-2 6-5 17-0 26-3 

Cat 51-1 6-5 17-1 25-3 

The figures for the nitrogen at least differ by more than the 
experimental error. The purification of the material is best 
effected by processes used in leather manufacture. After 
thorough cleansing the hide or skin is limed and unhaired, then 
washed, delimed, bated and scudded. By these means keratins, 
mucin, and elastin are removed, and what is left is regarded 
as collagen. On the laboratory scale it is desirable to treat 
the washed hide with trjrpsin in weakly alkaline solution in the 
presence of a little toluene to inhibit bacterial action. If the 
mixture is incubated at 37° C. the action of the trypsin is very 
rapid, and unhairing is soon accomplished. Elastin is also 
removed, according to recent work, as in the bating process. It is 
by this and similar methods that hide-powder (the purest form of 
collagen commercially obtainable) is prepared. The behaviour of 
coUagen with trypsin is interesting. It resists the action of the 
enzyme unless it has been previously treated with either pepsin, 
acid, or alkali. This has been adduced as evidence that coUagen 
has a ring, as distinct from an open-chain structure. The 
argument is, however, unconvincing, since many synthetic poly- 
peptides have been prepared which cannot be hydrolysed by 
trypsin. The view that collagen is an anhydride of gelatin is an 
old one, held by Hofmeister and others, and is partly based on 
the uncertain evidence of elementary analysis. Some have con- 
sidered that one molecule of gelatin loses one molecule of water, 
others that two molecules of gelatin lose one of water and thus 
become linked. Such a view has all the appearance of probability, 
and is supported firstly by the great resemblances in behaviour 
between gelatin and collagen, secondly by the ease with which 



148 PRINCIPLES OF LEATHER MANUFACTURE 

collagen is converted into gelatin, and lastly by the fact that 
gelatin when heated to temperatures above ioo° C. is gradually 
converted into an insoluble substance, which apart from lack of 
fibrous structure closely resembles collagen. The molecular 
weight of collagen is unknown, but certainly very high ; the 
combining weight of gelatin has been determined by Procter, and 
that of collagen may easily be the same (see Chapter X.). No 
boiling-point or osmotic-pressure determinations of the usual kind 
are possible, since collagen appears to be completely insoluble in 
water. Indeed, unless hydrolysis or other decomposition takes 
place, collagen will not dissolve in any reagent. Acids and alkalies 
swell collagen as they do gelatin, but no solution takes place unless 
the acid or alkali is fairly strong or assisted by heat or bacterial 
action. The view commonly held that hide-substance dissolves 
in 10 per cent, solutions of sodium chloride is probably erroneous. 
Many of the experiments quoted in its favour have been per- 
formed on hide incompletely purified, and in no case do the 
controls appear to have been sufficiently rigid. In other respects 
collagen behaves similarly to other proteins so far as its insolu- 
bility allows. It is acted upon by formaldehyde and the halogens, 
dehydrated or hardened by alcohol, strong solutions of ammonium 
sulphate, etc. The usual protein precipitants behave as tanning 
agents partly by chemical and partly by physical means. Hide 
gives all the colour reactions which can be carried out with solid 
protein, including the xanthoproteic and Millon tests. The first 
product of hydrolysis of collagen is gelatin, which is made by long 
boiling (many hours or even days) of purified hide, etc. Com- 
plete hydrolysis gives the amino-acids obtained from gelatin, 
enumerated in an earlier section of this chapter. The slow action 
or hydrolysis with cold dilute alkali or acid has not been studied 
from the chemical point of view. What is known with regard to 
gelatin will be described shortly, when it will be seen that one 
cannot take the identity of fresh and delimed hide for granted. 
The difference may be merely one of space arrangement of atoms 
within the molecule {i.e. stereo-chemical), but is probably real. 
The salt formation between collagen and acids or alkalies probably 
foUows closely the behaviour of gelatin, and is largely a matter of 
physical chemistry (see Chapter X.). That real combination 
takes place is proved by the impossibility of freeing limed hide 
from lime by washing with water. The amount present can be 
reduced from 4 or 5 per cent, to 1-5 per cent., but no further. 



THE CHEMISTRY OF HIDE 149 

Gelatin 

Gelatin does not occur in hide, but is the first hydrolytic pro- 
duct of collagen. Its interest for leather trades' chemists is very 
great, since it has many points of resemblance with collagen, and 
has been the subject of much chemical investigation. The purest 
varieties occurring commercially are colourless and transparent, 
of horny toughness, and of specific gravity about 1-3. Gelatin 
has no definite melting point, but begins to soften with decom- 
position at about 140° C. In cold water, alcohol, ether, or 
hydrocarbons, gelatin is insoluble, but swells to a transparent 
jelly in the first named. This swelling is one of the most im- 
portant properties of gelatin, and is especially marked in dilute 
acids and alkalies. A full discussion of the subject will be found in 
Chapter X. When the jelly is warmed it melts at temperatures 
from 30 to 40° C, and a solution of gelatin is obtained which sets 
again to a jelly on cooling if the concentration is above i per 
cent. Gelatin solutions should always be prepared by first 
swelling the gelatin, then pouring off the remaining water, adding 
further water as required, and finally melting by warming to a 
temperature not above 50° C. The last precaution is of im- 
portance if it is desired to dissolve the gelatin unchanged and 
avoid any further hydrolysis, since a solution heated to above 
70° C. will never return to its original state. The setting 
and melting temperatures of the jelly will be found to be 
permanently lowered, owing to the increased peptone content. 
Methods for the examination of glues and gelatines wiU be found 
in the special text-books. The determination of the melting 
point of the jelly may be mentioned here, as it is of particular 
importance. An angular fragment of jeUy should be put into a 
narrow glass tube attached to a thermometer, the whole in a 
beaker of water which is slowly heated until the jelly melts. Or the 
jelly may be allowed to set in an open capillary tube which is 
then treated as above, the temperature being noted when the 
water rises in the tube. The melting point varies very con- 
siderably with the quality of the gelatine, but is little affected by 
variations in concentration between 5 and 10 per cent. A 10 per 
cent, jelly of best hard gelatine melts at about 38° C. ; glue may 
melt at a temperature as low as 15° C. 

Ptcrification. — In much chemical work the best commercial 
gelatines have been used without any further purification, and 
with no regard to the origin of the material, i.e. whether 
from hide or bones. It is obviously too much to expect 
that commercial gelatine can be a pure substance, nor can it be 



150 PRINCIPLES OF LEATHER MANUFACTURE 

taken for granted that bone gelatine and skin gelatine are identical. 
The method of preparation by long boiling of hide makes purity 
in the chemical sense impossible, since gelatine formed at an 
early stage of the process will be subjected to the prolonged 
action of boiling water, and therefore notably hydrolysed. Actual 
examination proves that all commercial gelatines contain gelatoses 
and peptones, often in considerable quantities. Bogue has 
carried out analyses of some glues and gelatines with the following 
results: — 





Protein 


Proteose 


Peptone 


Amino-acid 




Nitrogen. 


Nitrogen. 


Nitrogen. 


Nitrogen. 


Russian isinglass 


91-0 


4-4 


4-5 


o-i 


Edible gelatine 


87-8 . 


II-3 


07 


0-2 


Hide glue . 


84-6 


12-4 


2-6 


0-4 


>> y> 


52-0 


38-6 


8-4 


0-9 


Bone glue . 


73-5 


16-4 


8-1 


2-0 


,, „ 


31-5 


50-6 


14-8 


3-0 


Peptone 


0-0 


33-2 


48-5 


i8-3 



Even the best French gelatine contains peptone. It is doubt- 
ful too whether the protein in commercial gelatine is a single" 
individual. The presence of chondrin has often been suspected, 
but no strict examination has been made. The usual way of 
purifying gelatin has been to wash it for several days in running 
water, preferably after treatment with dilute acid. Inorganic im- 
purities are thus largely removed by dialysis ; some peptone will 
also disappear at the same time. Miss D.J. Lloyd has lately purified 
gelatin by soaking it in successive changes of acid of concentrations 
so chosen that the final product is at its isoelectric point {i.e. 
Pjj=4-6). So treated the gelatin is milk-white and not trans- 
parent, and the ash content is very low (o-i per cent.). This 
method, however, does not appear finally to solve the problem. 
Dakin in the paper quoted above refers to the unsatisfactory 
nature of the present methods of purifying gelatin. 

Coagulation by Reagents, and Optical Activity. — Gelatin is 
completely precipitated from solution by half-saturation with 
ammonium sulphate ; indeed, from a prehminary experiment by 
the writer it appears to come down in slightly acid solution 
between 0-25 and 0-4 of complete saturation. It is also pre- 
cipitated by magnesium sulphate, and also in the presence of 
a little acid by sodium chloride. The coagulum is of course 
heavily contaminated with the salt used, which can be removed 
only by dialysis. Alcohol acts in the same way, and the 
amount precipitated is often 'determined as an index to the 



THE CHEMISTRY OF HIDE .151 

quality of commercial gelatine. Twenty-five c.c. of 10 per cent, 
gelatin solution is treated with 75 c.c. of absolute alcohol. On 
stirring it becomes firmly attached to the rod and the sides of the 
beaker, where it may be washed with dilute alcohol, cold water, 
and finally dried and weighed. A French gelatine has yielded 
98-6 per cent, of coagulum by this method, whilst glues may give 
no more than 60 per cent. Gelatin jellies shrink and harden by 
the same treatment. Alcohol will reduce a jelly to a horny mass 
by the withdrawal of water. 

While discussing the physical behaviour of gelatin, it may be 
mentioned that its solutions are strongly laevorotatory to polar- 
ised light. At 30° C. the specific rotation is about —130°, but 
the temperature and reaction of the solution exert great influence 
on the value found. In common with other proteins, gelatin is 
racemised by the slow action of dilute alkali at moderate tempera- 
tures, i.e. N/2 or N/4 caustic soda at 30 to 40° C. In other words, 
the optical activity diminishes to a. fraction of its original value. 
Dakin has explained this as due to a change in the mode of 
linkage at some —CO . NH— groups. This change is known as 
the " keto-enol " transformation, and may be formulated thus : 

~C NH -> =C N = . 

Dakin is of the opinion that only those carbon atoms are un- 
affected which are contained in amino-acids at the ends of chains. 
Racemisation does not appear to be due to hydrolysis. 

Reactions of Gelatin. — From the point of view of colour tests 
and precipitation reactions gelatin is hardly a typical protein. 
Its solutions fail to give the glyoxylic, Millon, and sulphur tests 
(in spite of the presence of sulphur), and produce only a slight 
xanthoproteic reaction. It follows that the amino-acids trypto- 
phane, tyrosine, and cystine are absent, and that phenylalanine 
is only present in small quantity. This is borne out by the 
results of complete hydrolysis. Gelatin is precipitated by tannin 
and phosphotungstic acid, but not by normal lead acetate or 
ferrocyanic acid. This behaviour naturally suggests comparison 
with the albumoses. 

The tannin-gelatin reaction is of such importance that some 
space must be devoted to it. All tannins precipitate gelatin even 
in very dilute solution, provided that excess of gelatin is avoided, 
as otherwise the precipitate is dissolved. The gelatin may behave 
here as a protective colloid. The most sensitive reaction is 
obtained with gallotannin, obtained from gall-nuts. Some sub- 



152 PRINCIPLES OF LEATHER MANUFACTURE 

stances such as alum, basic chrome Hquor, and metaphosphoric 
acid intensify the reaction, others, positively charged colloids, 
retard it. In any case the reaction will only take place in an 
acid solution. If a series of trials with gallotannin and gelatin 
be made, beginning with acid solutions and adding small increasing 
quantities of alkali, the reaction will ultimately fail with rather 
surprising suddenness. There has been much debate as to the 
nature of the precipitate. Different workers have obtained the 
most widely varjdng results for its composition, and it was left to 
J. T. Wood to show that a precipitate of constant composition 
could only be obtained in the presence of a sufficient excess of 
tannin. If this excess were provided (6 parts tannin to i part 
gelatin), then i grm. of gelatin combined with 2-4 grm. of 
tannin, and variation in the concentration of the solutions was 
found to have very little effect. This result is sufficient to dis- 
prove any hypothesis based on so-called adsorption in which 
concentrations are the dominant factor and absolute amounts of 
no importance. The most probable explanation is that we have 
the mutual neutralisation of oppositely charged colloids, since 
when the reaction takes place, in acid solution only, we have the 
gelatin positively and the tannin negatively charged. This is 
followed by precipitation. In alkaline solution, when both 
colloids bear a negative charge and no neutralisation is possible, 
there is no precipitation. It will be noticed that this is strictly 
analogous to an ionic reaction. Instead of the mutual neutralisa- 
tion of oppositely charged ions, the reaction is between large 
colloidal particles bearing charges probably derived from attached 
ions. 

Various salts diminish the solubility of gelatin in water and 
raise the melting point of jellies. Such are alum, chrome alum, 
and basic chrome salts. Potassium dichromate behaves similarly 
when aided by the action of light. The dichromate is reduced 
and tans the gelatin. This behaviour is utilised in making 
cements. Formaldehyde also insolubilises gelatin. Other salts 
either raise or lower the melting point of gelatin jellies, but their 
effect is physical rather than chemical. 

Hydrolysis. — The complete hydrolysis of gelatin has already 
been referred to and a list given of the products obtained. We 
ought to add a little about the phenomena of partial hydrolysis, 
though the work in this field has been very scanty. Well-defined 
peptones have, however, been isolated by Siegfried by his iron 
alum method after the action of pepsin or trypsin. These 
peptones have been subjected to further hydrolysis with interest- 
ing results. For instance, the peptone obtained by the action of 



THE CHEMISTRY OF HIDE 153 

trypsin was heated with 12-5 per cent, hydrochloric acid at 38° C. 
After 113 hours the optical rotation had fallen from —8-4° to 
—5-0°, and remained constant for 214 hours. From this solution 
a body was obtained which Siegfried calls a kyrin, and which, 
on hydrolysis, yielded only arginine, lysine, and glutamic acid. 
Further work on this body proved beyond reasonable doubt that 
it was an actual tripeptide, the elementary analysis giving figures 
almost exactly corresponding to theory. This result is an 
important contribution to knowledge of the structure of gelatin, 
seeing that it is less likely under the circumstances of the experi- 
ment that the tripeptide was formed from amino-acids previously 
split off. 

Elastin, etc. 

Elastin. — This protein constitutes the yellow elastic fibres of 
the corium, and is of a stable character. It is scarcely possible 
to isolate it from hide, and our knowledge of elastin is mainly 
derived from that of the tendons and ligaments, particularly 
ligamentum nuchcB, the very strong and thick ligament in the 
neck of cattle, etc. This elastin is assumed without proof to 
be identical with that of the elastic fibres. In chemical 
properties elastin shows considerable similarity to keratin, being 
very resistant to boihng water and only slowly attacked by hot 
acids and alkalies. The products of complete hydrolysis have 
been given, and in this respect elastin does not resemble the 
keratins. The elastic fibres are apparently unable to combine 
with tannin. It has been stated recently by several writers that 
the elastic fibres tend to disappear in the bating process, being 
acted upon by the trypsin or other enzymes. This is a very 
attractive view of the bating process, and has been strongly 
supported by published photomicrographs. On the other hand, 
it has been objected that elastin from ligamentum nuchce is 
resistant to trypsin. These facts are not inconsistent, since 
the identity of the two elastins is not certain, and also the elastic 
fibres before being bated are limed, which may make a very great 
difference. 

Keratins. — The proteins of the hair and epidermis belong to 
the class of keratins. These substances are quite insoluble in 
water, though somewhat softened by it, and are very resistant 
to boiling water except under pressure. If heated with water 
at 160° C. for a long period the keratin is broken up, dissolving 
for the most part and evolving sulphuretted hydrogen. The 
softer keratins are hydrolysed fairly easily by caustic alkali, 
but hair and horn require strong hot solutions. The action 



154 PRINCIPLES OF LEATHER MANUFACTURE 

of sodium sulphide is remarkable ; harder and softer keratins 
alike are easily attacked. As is well known, quite small 
quantities materially assist the unhairing action of lime-liquors. 
Keratins give all the protein reactions except when insolubility 
is an obstacle, since all the amino-acids giving the reactions are 
present, e.g. tyrosine, etc. The most striking feature in the 
chemistry of keratins is the high content of sulphur, due to the 
presence of large quantities of the amino-acid cystine. This 
amino-acid is decomposed by alkalies producing sulphide, which 
in the case of lime-liquors soon oxidises to sulphate and thio- 
sulphate. A solution of keratin in alkali gives a precipitate 
(probably of proteoses) on acidification. This precipitate, which 
usually appears when a lime-liquor is neutralised, has been pro- 
posed as a filling material for leather, but without any very 
general success. 

Mucins. — The mucins are conjugated proteins termed gluco- 
proteins, since the protein is united with a carbohydrate group. 
Hence on hydrolysis with acids the solution obtained contains 
sugar and is able to reduce Fehling's solution. Mucins are easily 
soluble in dilute alkali, and are consequently removed from hide 
by the liming process. They can be precipitated by acetic acid 
from alkaline solution (provided that no hydrolysis has taken 
place), since they are insoluble in excess of acetic acid. The 
protein comes down as a stringy mass. All the characteristic 
protein reactions are given by mucins. One of the most character- 
istic features of mucins is the great viscosity of their solutions. 

Casein. — This protein is never found in hide, but is interest- 
ing in leather manufacture, since it is a constituent of various 
finishes and cements. It is the principal protein of milk, belongs 
to the phosphoprotein class, is of a more pronounced acidic 
nature than most proteins forming fairly well-defined caseinates. 
In milk it probably exists as calcium caseinate, and is usually 
prepared from separated milk by acidification or treatment with 
rennet, when it precipitates at once. Casein dissolves in alkalis 
and less easily in acids. Few proteins have been so thoroughly 
investigated as casein, but the minor importance of the protein 
to the leather industry does not justify any discussion of the 
results here. 

Albumins. — The albumins are soluble in water, dilute salt 
solutions, acids, and alkalies, and are the typical coagulating 
proteins. In neutral solution they are not precipitated by 
saturation with magnesium sulphate or half-saturation with 
ammonium sulphate, but in acid solution some precipitation 
occurs. FuU saturation with ammonium sulphate causes com- 



THE CHEMISTRY OF HIDE 155 

plete precipitation. All the protein reactions are given by the 
albumins. Heat-coagulated albumin somewhat resembles keratin, 
being quite insoluble unless partially hydrolysed by acids or 
alkalies. Egg-white, the source of egg-albumin (ovalbumin) , is a 
slightly alkaline fluid, containing about | of solid matter and | 
water. The protein is contained in membranes which are broken 
up by whisking. The albumin is here associated with a globulin, 
a coagulable protein which can be removed by half-saturation 
with ammonium sulphate. Serum or blood albumin is associated 
with a globulin in serum, the fluid left when the corpuscles are 
removed by centrifuging defibrinated blood. In preparing solu- 
tions of albumins care must be taken not to allow the temperature 
to reach the coagulation point. 



Books for Reference 

S. B. ScHRYVER, General Characters of the Proteins. Long- 
mans & Co. 

R. H. A. Plimmer, Chemical Constitution of the Proteins. 
Parts I. and II. Longmans & Co. 

S. W. Cole, Practical Physiological Chemistry. Heffer & 
Co., Cambridge. 

R. H. A. Plimmer, Practical Organic and Bio-Chemistry. 
Longmans & Co. 

T. B. Robertson, Physical Chemistry oj the Proteins. Long- 
mans & Co. 

W. Pauli, Kolloidchemie der Eiweisskorper. Theodor Stein- 
kopff, Dresden. 



CHAPTER XII 

SOAKING AND SOFTENING OF HIDES AND SKINS 

As has been explained in the last chapter, hides and skins come 
into the hands of the tanner either uncured (" green "), as they 
are taken off the animal, preserved with salt or some other 
antiseptic, dried, or " dry-salted," in which both methods are 
combined. His object in each case is to remove blood and dirt, 
and to restore the hide to its soft and natural condition ; but the 
treatment required varies much with the state of the hides. 

Fresh hides merely require cleansing from blood and dirt. 
This is necessary because the blood causes bad colour from the 
iron contained in its haemoglobin, and both blood, lymph, and 
adhering dung are sources of putrefaction, which ultimately 
attacks the grain and fibrous structure of the hide. Hence 
washed hides keep better than unwashed. Cold water is most 
desirable, as checking putrefaction. If the water is much over 
10° C, or if it is charged with organic matter and ferment-germs, 
or if, as is too generally the case, the hides are in a partially 
putrid state when received, the time of soaking must be reduced 
as much as possible, and it may be necessary to sterilise the 
water with carbolic acid or creolin (pp. 21, 28). In such cases the 
use of a wash-wheel, or tumbler, is very desirable, rapidly cleansing 
the hides and removing adhering dung, which interferes with the 
liming, and is a serious cause of damaged grain. The American 
pattern of wash-wheel shown in fig. 25 is very suitable for the 
purpose. In no case is it desirable to allow green hides to lie 
for more than a few hours in water ; and unwise treatment at 
this time is the cause of many troubles, which are only detected 
at later stages, and which are very difficult to trace to their 
source. " Weak grain," in which the grain-surface (p. 56) is 
destroyed, and which tans a whitish colour; "pricking," orperfora- 
tion of the grain with small pinholes, which may go on to "pitting " 
with larger holes, and a general weakening of fibre, with softening 
and needless loss of weight, are among^ these results. An 
instructive instance may be quoted. A large tanner found that 
his curried leather was affected with small 'spots and rings of 
darker colour, which rendered it quite unfit for staining, and which 
reappeared even when the leather was buffed. When finished as 

156 



SOFTENING OF HIDES AND SKINS 157 

black grain these spots had a tendency to " spue/' or rise as little 
pimples of resinous matter. Before the leather was stuffed no 
defect was noticeable to the eye, but either then, or on stripping 
the grease by a solvent, they cotild be seen under the micro- 
scope as lighter patches of open and porous grain which absorbed 
more than their share of fat. During the tanning process they 
could hardly be detected, but in the first colouring they appeared 
for a few hours as blackish specks almost exactly like those 
caused by particles of iron or iron-rust. By careful observation 
they were traced back to the limes ; specimens of the limed hide 
were submitted to Director Eitner, who identified the defect as 
" Stippen," caused by a species of bacteria, which cannot subsist 
in limes, and which therefore must have been in the soaks. 
These, which had been somewhat neglected from pressure of 
work, were cleaned out and sterilised with creolin solution, and 
the mischief ceased. It is worth noting that the tanner dated 
the beginning of the trouble from the soaking of some " Spanish " 
horse-hides, which may have introduced the infection. Several 
very similar cases have come under the writer's notice. 

It is not absolutely necessary to soak fresh hides or skins at all 
before liming, and where the water is scarce or unfavourable, or 
the skins tainted or " slipping " hair, it is best to pass straight 
into a weak lime. In this case the limes must be worked in 
shifts (see p. 179) and the whole of the oldest liquor run away and 
the hides rapidly changed into a fresh lime, or the limes will 
become so charged with organic matter and bacteria that the 
hides will cease to plump, and may even putrefy. 

Salted hides and skins require more soaking and more thorough 
washing than fresh ones, as it is not only advisable to remove the 
salt, but to soften and plump the fibre which has been dehydrated 
and contracted by salting. If goods with salt in them are taken 
into limes they will not plump properly, and creases and wrinkles 
(drawn grain) are formed which no after-treatment will remove. ■*■ 
This is especially important in sole leather. In deciding on a 
method, we must bear in mind that salt is easily soluble, and 
diffuses rapidly into water or weaker solutions, but slowly into 
strong, and not at all into saturated ones. It may also be noted ■ 
that though salt is not a true disinfectant (p. 21), salted hides 

^ This opinion is generally held by tanners, and there is no doubt that 
salt does oppose the plumping of hide in caustic soda solutions, though 
not nearly so powerfully as in acids (as in the tan liquors). It is not 
improbable that the wrinkles and creases referred to are more due to want 
of sufficient swelling and softening in the water-pit than to the presence 
of the salt. Salt may increase swelling in unsharpened limes. 



158 PRINCIPLES OF LEATHER MANUFACTURE 

are much less prone to putrefaction than fresh ones, and therefore 
a longer soaking may be safely given. 

These conditions point to the desirabihty of free exposure to 
water, attained by suspending, handling frequently, or tumbling, 
and repeated changes to remove the salt. The degree of removal 
of salt is easily determined by the estimation of CI in the last 
wash-water {L.I.L.B., p. i8). American tanners universally soak 




Fig. 25. — American Wash-wheel. 



wet-salted hides three or four days with as many changes of M^ater, 
and frequently finish by a few minutes in a wash-wheel. Any 
washing tumbler may be used ; but the cheap and simple con- 
struction of the American wash-wheel will be easily understood 
from fig. 25. The sides are open, so that hides can be put in or 
removed between the spokes. The rim of the wheel is generally 
perforated for the escape of water, which is supplied by a pipe 
passing through the axis ; and the wheel is often driven by a 
chain or rope round its circumference. No severe mechanical 
treatment, such as " stocking," is necessary or desirable for green 
or salted hides. 



SOFTENING OF HIDES AND SKINS 159 



Dry and dry-salted hides require much longer soaking than wet- 
salted, the amount naturally depending on the thickness of the 
hide and the character of drying. Even thin skins when strongly 
dried require considerable time to soften and swell the fibres, 
although they soon become wet-through and flexible. Many 
different methods of soaking have been employed. Sometimes 
hides are suspended in running water ; sometimes laid in soaks 
which may be either renewed, or allowed to putrefy ; sometimes 
in water to which salt, borax, or carbolic acid has been added 
to prevent putrefaction ; and more recently weak solutions of 
caustic soda, sulphide of sodium, or sulphurous acid have been 
used with much success. 

The first of these methods, were it desirable, is rarely possible 
in these days of River Pollution Acts ; of the others, it is difficult 
to say which is better, since the treatment to be adopted varies 
with the hardness of the hide and the temperature at which it 
has been dried. The great object is to thoroughly soften the 
hide without allowing putrefaction to injure it. As dried hides 
are often damaged already from this cause, either before drying, 
or from becoming moist and heated on shipboard, it is frequently 
no easy matter to accomplish this. The fresh hide, as has been 
seen, contains considerable portions of albumin, and if the hide is 
dried at a high temperature, this may become wholly or partially 
coagulated and insoluble. The gelatinous fibre and the coriin (if 
indeed the latter exists ready formed in the fresh hide) do not 
coagulate by heat, but also become less readily soluble. Gelatin 
dried at 130° C. can only be redissolved by acids, or water at 
120° C. Eitner ^ experimented with pieces of green calf-skin of 
equal thickness, which were dried at different temperatures, with 
results given in the following table : — 



Sample. 

I. 
II. 

III. 

IV. 



Tempera- 
ture of 
Drying. 

15° c. 

22° C. 

35° C. 
60° C. 



Remarks. 



In vacuo 
In sun 

In drying- 
closet 



Time of 
Softening 
in Water. 

24 hours 
2 days 



Remarks. 

r Without "1 
: mechanical 
[_ work J 
Twice worked 



Refused to soften suffi- 
ciently for tanning. 



Dissolved 
by Salt 
Solution. 

1-68% 

1-62% 

0-15% 
traces 



Hence it is evident that, for hides dried at low temperatures, 
short soaking in fresh and cold water is sulficient, and, except in 

^ Gerber, 1880, p. 112. 



i6o PRINCIPLES OF LEATHER MANUFACTURE 

warm weather, there would be httle danger of putrefaction. With 
harder drying longer time is required, and more vigorous measures 
may be necessary. A well-known ta.nner recommended a brine 
of 30° to 35° barkometer (sp. gr. 1-035, or about 5 per cent, of 
NaCl). This has a double action, not only preserving from putre- 
faction, but dissolving a portion of the hide-substance, which is 
undoubtedly a loss to the tanner, though it is questionable if there 
is any process which will soften overdried hides without loss of 
weight ; since even prolonged soaking in cold water at a tempera- 
ture which is too low to allow of putrefaction taking place will 
dissolve a serious amount of the hide-constituents. Chlorides, 
however, do not seem well adapted for the purpose in view, 
from their weak antiseptic power and tendency to prevent 
swelling. Jackson Schulz advised the use of water at 80° F. 
for soaking during the winter months. Water containing a small 
quantity (o-i per cent.) of carbolic acid has been recommended 
for the purpose, and will prevent putrefaction, while it has no 
solvent power on the hide, but, on the contrary, tends to coagulate 
and render insoluble albuminous matters. Borax has been pro- 
posed for the same purpose, and, in i per cent, solution, certainly 
prevents putrefaction, and has considerable softening power, but 
is far too costly. Other methods of chemical softening are 
described on p. 161. 

For some descriptions of hides, and notably for India kips, 
putrid soaks were formerly much employed, the putrefactive 
action softening and rendering soluble the hardened tissue. In 
India the native tanners soften their hides in very few hours by 
plunging them in putrid pools, into which every description of 
tannery refuse is allowed to run. Putrefactive processes, how- 
ever, are always dangerous, as the action, through changes of 
temperature, or variation in the previous state of the liquor, is 
apt to be irregular, and either to attack one portion of the hide 
before another, or to proceed faster than was expected. Hides 
are also frequently more or less damaged by putrefaction and 
heating during the process of cure, and these damages are 
accentuated in a putrid soak. Hence hides in the soaks require 
constant and careful watching, and the goods must be withdrawn 
as soon as they are thoroughly softened, for the putrefaction 
is constantly destroying as well as softening the hides. It is 
possible that putrefactive softening is less injurious to kips, and 
such goods as are intended for upper leather, than to those for 
sole purposes, as it is generally considered necessary in the 
former case that a good deal of the albumin and interfibrillary 
matter be removed, and that the fibre be well divided into its 



SOFTENING OF HIDES AND SKINS i6i 

constituent fibrils for the sake of softness and pliability ; and 
thus the putrid soak, if acting rightly, accomplishes part of the 
work which would afterwards have to be done by the lime and 
the bate, as the actual fibre of the hide seems less readily putres- 
cible than the softer cementing substance. 

Putrefaction is caused, as we have seen, by a great variety of 
living organisms, each of which has its own special products and 
modes of action. It is quite possible that, if we knew what precise 
form of putrefaction was most advantageous, we might by appro- 
priate conditions be able to encourage it, to the exclusion of 
others, and obtain better results than at present. Putrid soaks 
(in the old sense) are, however, disused in the present day by all 
enlightened tanners, as it is recognised that the risks outbalance 
the advantages, and when dry-salted hides are worked, the soluble 
salts of the cure accumulate in the soaks to an injurious extent. 
The modern method, where no chemicals are used, is to give one 
fresh water at least to each pack of hides or skins. Even in this 
case considerable putrefaction takes place where the soaking 
occupies seven to fourteen days, as is the case with kips and hides, 
and there is no doubt that the use of chemical and antiseptic 
methods of soaking will ultimately be generally adopted, both on 
technical and sanitary grounds. 

The use of dilute acids for softening has much to recommend 
it, their power of causing the fibre to swell and absorb water 
being at least equal to that of the alkalies, while few, if any, 
putrefactive bacteria can thrive in an acid liquid. Very dilute 
sulphuric acid has been used with success to dissolve the alkaline 
" plaster " of East India kips (p. 39), It has considerable dis- 
infectant power (p. 23), but its action on the hide-fibre is 
undesirably strong. 

Sulphurous acid is much more suitable. Its use for this pur- 
pose was patented by Maynard, along with a number of other 
possible uses, but the patent has long lapsed, and he did not 
seem to have succeeded in introducing it into practice. Experi- 
ments at the Yorkshire College (now Leeds University), and also 
at a tannery on a manufacturing scale, have shown that the 
method is capable of excellent results. The hides are soaked for 
twenty-four to forty-eight hours in a solution of sulphurous acid 
containing about 2 per cent, of SOg (for manufacture, compare 
p. 24 ; for testmg, L.I.L.B., pp. 16, 37), and are then transferred 
to water, where they swell freely to their full thickness. They 
may be either limed at once, or first neutralised with dilute caustic 
soda, ammonia, or sulphide of sodium, which, for dressing leather, 
is perhaps desirable. No putrefaction takes place, even if they are 

II 



i62 PRINCIPLES OF LEATHER MANUFACTURE 

retained for a considerable time in water, and the acid has little 
or no solvent effect on the hide-fibre, the strength of which is 
well preserved. The liming, however, must either be conducted 
with the aid of sodium sulphide or in old limes, since the sterile 
condition of the hides renders liming in fresh lime very slow 
[cp. p. 183). For experimental purposes a ^ per cent, solution 
of Boakes' " metabisulphite of soda " may be used, to which 
I per cent, of concentrated sulphuric acid previously diluted 
with water is gradually added during the soaking, the hides being 
first withdrawn. For regular work it will be found niuch cheaper 
to manufacture the acid on the spot by burning sulphur. 

The use of solutions of caustic soda (i to 2 parts per 1000), or 
of sodium sulphide {i\ to 5 parts per 1000) as suggested by 
Eitner, seems at present likely to supersede all other methods of 
softening from their simplicity and safety. Twenty-four to forty- 
eight hours in either of these solutions, which may if necessary 
be followed by a short soak in plain water, seem sufficient to 
soften either kips or hides. Experiments at Leeds University 
have shown that solutions of this strength have little or no solvent 
action on the hide-fibre, but promote its swelling in water so 
effectively that no mechanical softening is needed (though a slight 
drumming is advantageous), while putrefaction is almost entirely 
prevented, so that the solution may be repeatedly used if kept up 
to its original strength, which is easily determined with standard 
acid and phenolphthalein (see L.I.L.B., p. 17). Neither caustic 
soda nor sodium sulphide have any injurious effect on liming, 
though it may prove somewhat slower than with the older 
methods, where the epidermis was partially destroyed by the 
action of putrid ferments. The dilute solutions used are not 
only less injurious to the hide than those of greater strength, but 
they are also more effective in softening. Eitner {Gerber, 1899, 
p. 584) states that when using a solution of caustic soda of i part 
in 1000 strength, the time required to soften some hides was 
only two days, as against three days for a sodium sulphide liquor 
and four days for pure water, and that with the soda solution 
only about 0*6 per cent, of the hide-substance of the skin was 
dissolved out, whilst when sodium sulphide was used it was 07 
per cent., and with pure water alone no less than i'9 per cent, 
was lost by solution. 

The use of moderately warm water (40° C.) in a drum is quite 
successful in rapidly softening sound hides after they have 
previously been soaked for some days in cold water ; but if they 
are tainted in the cure, it is very apt to intensify the mischief. 
Hides which have partially putrefied internally, or which have 



SOFTENING OF HIDES AND SKINS 163 

been exposed to a hot sun while the interior is still moist, are 
very apt to appear sound while dry, but to blister or go to 
pieces from the destruction of the internal fibres as soon as they 
are limed, and this in spite of even the most careful treatment. 
For tainted hides, caustic soda is probably preferable to sodium 
sulphide. 

Many chemicals have been patented for softening hides. 
Sulphide of arsenic is said to be in use, and if dissolved in caustic 
soda solution would differ little in its effect from ordinary sul- 
phide of sodium. Saltpetre has also been employed, but its 
effect, if any, was probably merely antiseptic. Ordinary sodium 
carbonate has been used, but is less effective than caustic soda, 
and must be used in stronger solution. Gas liquor and mixtures 
of this with tar and water were patented by Barron, and probably 
the first would soften by virtue of its ammonia and sulphides, while 
tar contains carbolic acid. Probably the most absurd mixture 
of all was patented by Berry, which consisted of ^ bucket of 
slaked lime, J bucket of wood-ashes, 12 lbs. of potash, 5 lbs. of 
oil of vitriol, and 4 lbs. of spirit of salt ! 

Beside merely soaking the hides, it is sometimes necessary to 
work them mechanically to promote their softening ; this was 
formerly accomplished by " breaking over " the hides on the 
beam with a blunt knife. This process is still in use for skins 
of many sorts, but for the heavier classes of leather was usually 
superseded or supplemented by the use of " stocks," or drums. 
The former consist of a wooden or metallic box, of peculiar shape, 
wherein work two very heavy hammers, raised alternately by 
pins or cams on a wheel, and let fall upon the hides, which they 
force up against the curved end of the box with a sort of kneading 
action. The ordinary form of this machine is shown in fig. 26. 
A more modern form, which seems to possess some advantages, 
is the American " double-shover," or " hide-mill," seen in fig. 
27. " Crank stocks," similar in form to the faller stocks, but 
driven by cranks, are sometimes used for softening, but are 
better adapted to lighter uses. 

The number of hides which can be stocked at once naturally 
varies with the size of both hides and stocks, but should be such 
that the hides work regularly and steadily over and over. The 
whole number should not be put in at once, but should be added 
one after another, as they get into regular work. The duration 
of stocking is ten to thirty minutes, according to the condition 
and character of the hides. Hides should not be stocked until 
they are so far softened that they can be doubled sharply without 
breaking or straining the fibre. After stocking, they must be 



i64 PRINCIPLES OF LEATHER MANUFACTURE 

soaked again for a short time, and then be brought into an old 
lime. A small quantity of sodium sulphide added to the soaks 
or in the stocks has been recommended as of great value in soften- 




FiG. 26. — Faller Stocks. 




Fig. 27. — American Hide-mill. 



ing obstinate hides, and probably with justice, from its well- 
known softening action upon cellular and horny tissues. 

Tumbler drums of various forms may also be used with good 
effect for softening purposes, especially for skins, and are much 
less detrimental than stocking, both as regards the weight and 
quality of the goods. 



SOFTENING OF HIDES AND SKINS 165 

For sole leather, and even for kips, the use of stocks has in 
recent years been entirely discarded by many of the more 
advanced tanners. If mechanical work is required at all the 
drum is preferred, and is sometimes employed after a few days' 
liming, the goods being first merely softened in fresh water. The 




Fig. 28. — Drum for Washing or Tanning. 



use of caustic soda, sodium sulphide, or sulphurous acid usually 
renders mechanical softening unnecessary. 

The drums employed are in principle like a barrel-churn, and 
are large cylindrical wooden chambers 6 to 12 feet in diameter, 
and fitted inside either with shelves like the floats of a water- 
wheel, or with rounded pegs on which the hides faU. The 
American wash-wheel figured on p. 158 is a machine of this kind, 
and one of a more elaborate description is shown in fig. 28. 
Drums are not only used for softening, but for tanning, dyeing, 
and many other purposes in leather manufacture. It is advan- 
tageous to be able to reverse the direction of their rotation to 
prevent the rolling up of the hides. 



CHAPTER XIII 

DEPILATION 

After the softening and cleansing of the hide or skin is com- 
pleted, and before proceeding to tan it, it is usually necessary to 
remove the hair or wool. The earliest method of accomplishing 
this was by means of incipient putrefaction, which attacks in the 
first instance the soft mucous matter of the epidermis, and thus 
loosens the hair without materially injuring the true skin. This 
loosening of the hair often takes place accidentally in hides 
which have been kept too long without salting, and is known 
as " slipping," and is apt to be accompanied by some degree of 
injury to the grain. The old method of loosening the hair by 
putrefaction, or, as it is generally called, " sweating," was to 
lay the hides in piles, usually in some warm and damp place. 
Occasionally a slight preliminary salting was given to prevent 
too much putrefaction of the hide. The action in this case, 
■ h9wever, was very irregular, and it has been quite abandoned 
in all civilised countries. 

A method which is still used to some extent in America, princi- 
pally on dry hides, is to hang the hides in a closed chamber, 
generally called a " sweat-pit," fig. 29, but usually constructed 
above the ground-level, and protected from sudden changes of 
temperature by double walls, or by mounds of earth. The 
hides are hung in the sweat-pit, in small chambers each capable 
of holding 50 or 100 hides. The temperature is kept at about 
15° to 20° C, the air being warmed, if necessary, by the admission 
of steam below a perforated floor, or cooled by a shower of water 
from sprinklers, so arranged as not to play directly on the skins, 
and is thus always kept saturated with moisture. Little if any 
ventilation is allowed, and a large quantity of ammonia is given . 
off from the decomposition of the organic matter, and no doubt 
contributes to the solution of the epidermis and the loosening of 
the hair, as the writer has found that ammoniaccil vapours alone 
very speedily produce this effect. It may here be suggested 
that the use of ammoniacal gas for sheep-skins deserves practical 
trial, as the loosening of the wool is very rapid, and no injury 
is done either to it or to the pelt. The ammonia could be re- 
covered by passing the air of the chamber through an >acid 

166 



DEPILATION 



167 



" scrubber," or probably transferred to a second chamber by 
displacement with cold air, as ammonia gas is lighter than air 
and would float on colder air admitted at the bottom. Ammonia 
is also formed during the process. 

After four to six days of this treatment the hair is sufficiently 
loosened to be removed by working the skin over the beam with 
a blunt knife, or by means of the stocks or hide-mill (see p. 164). 
Great care and watchfulness are required to avoid injury to the 
grain by putrefaction. 

The hide is in a slimy and completely flaccid and " fallen " 
condition, and some trouble is occasioned by the hair being 




Fig. 29. — Sweat-pit. 



worked into the flesh by the hide-mill, to obviate which a slight 
liming is frequently given after the sweating. Hides which have 
been unhaired in this way require to be swollen by acid. in the 
liquors in order to produce a satisfactory sole leather, as the 
sweating process does not swell or split up the fibres. 

In some European tanneries a similar process, but at a higher 
temperature, is employed, and it is also largely used for sheep- 
skins under the name of " staling," but in this case is sometimes 
conducted in a very rude and primitive manner, and frequently 
with the result of considerable injury to the pelt. This injury 
is not always bacterial. A case is described by P. Hampshire ^ 
in which great destruction to pelts was caused by nematode 
wprms. The process is adopted because the wool on these skins 
is of greater value than the pelt, and is less injured than by 
liming, but is being largely superseded by painting with sulphide 
mixtures. 

The great objection to the sweating process, however care- 
fully conducted, is the liability of putrefaction to attack the skin 
1 J.S.L.T.C., 1921, 5, p. 20. 



i68 PRINCIPLES OF LEATHER MANUFACTURE 

itself, causing " weak grain." Its most advantageous use is for 
sole leather, as, although the solution of the hide-substance may 
not be very much less than in the case of liming, the dissolved 
matter remains in the hide instead of being washed out, and 
being fixed by the tannin, contributes to the solidity of the 
leather. 

In England, lime is the agent almost universally employed for 
unhairing (now almost invariably with the addition of alkaline 
sulphides), though every tanner admits its deficiencies and dis- 
advantages. It is hard, however, to recommend a substitute 
which is free from the same or greater evils, and lime has one 
or two valuable qualities which will make it very difficult to 
supersede. One of these is that, though it inevitably causes loss 
of substance and weight, it is also impossible, with any reasonable 
care, totally to destroy a pack of hides by its use ; which is by 
no means the case with some of its rivals. Another advantage 
is that, owing to the very limited solubility of lime in water, it is 
of comparatively small consequence whether much or little is 
used ; and even if the hides are left in a few days longer thaii 
necessary, the mischief, though certain, is only to be detected by 
careful and accurate observation. With all other methods, exact 
time and quantity are of primary importance, and it is not easy 
to get ordinary workmen to pay the necessary attention to such 
details. Again, the qiialities of lime, its virtues and failings, 
have been matter of experience for hundreds of years, and so 
far as such experience can teach, we know exactly how to deal 
with it. A new method, on the other hand, brings new and 
unlooked-for difficulties, and often requires changes in other 
parts of the process, as well as in the mere unhairing, to make 
it successful. As our knowledge of the chemical and physical 
changes involved becomes greater, we may look to overcoming' 
these obstacles more readily. 

The universal source of lime is chalk or limestone, which con- 
sists of calcium carbonate, and from which the carbon dioxide 
is driven off by burning in a kiln. Many limestones, however, 
are far from being pure calcium carbonate, but contain large 
proportions of magnesia, iron, and alumina, the latter perhaps 
originally deposited in the form of clay with the sediment from 
which the stone was formed. Such clay limestones when burnt 
yield natural cements, like oolite and other " hydraulic " limes, 
which are capable of setting even under water. The presence 
of magnesia and clay is injurious, not only by diminishing the 
amount of lime present, but by making the lime much more 
difficult to slake ; and iron oxide, though quite insoluble, may 



DEPILATION 169 

become mechanically fixed in the grain of the hide, and may 
be the cause of subsequent stains. The burning of lime in the 
kiln is probably not quite so simple an operation as the equations 
of the text -books would suggest. By mere heating the carbonate 
can, it is true, be decomposed, but to do this completely a good 
white heat is required, which is rarely attained in practical 
burning, and it is probable that at least a part of the carbon 
dioxide present is reduced to carbon monoxide by the com- 
bustible fuel-gases, and so separated from the lime, for which 
it has no affinity. Carbon monoxide is the cause of the intensely 
poisonous character of lime-kiln gases, the pure dioxide being 
irrespirable, but not strictly poisonous. 

Quicklime, CaO, on coming in contact with water, combines 
with it with the evolution of considerable heat, becoming slaked 
or converted into hydrate, Ca(0H)2. This change takes place 
rapidly and easily when the lime is light and porous, such as is 
obtained by the burning of chalk or good limestone at a low 
temperature ; but if it has been too intensely heated or " over- 
burnt," or contains silicates or other salts which fuse at the 
temperature of the kiln, a compact lime is formed which slakes 
with difficulty and extreme slowness, thus being lost to the 
tanner, or leading to the still more serious result of burning holes 
in the hides by the heat produced by slaking in contact with 
them. It is stated by Le Chatelier ^ that for dense limes twenty- 
four to forty-eight hours is frequently required for complete 
slaking in the cold, while magnesia is still more obstinate, months 
being sometimes necessary for the complete hydration of hard- 
burnt samples; and mixtures of lime and magnesia are inter- 
mediate in their character. Slaking is greatly assisted by heat, 
even heavily burnt magnesia being hydrated in about six hours 
at 100° C. Slaking is also much more rapid in a dilute solution 
(2 per cent.) of calcium or magnesium chloride. From these 
facts it is easy to deduce the reason why a suitable quantity of 
water, neither too much nor too little, is desirable for the rapid 
and effectual slaking of lime. If too little is used, the lime is 
only partially slaked, and it is not easy for further portions of 
water to gain access to the interior of the powdery mass. On 
the other hand, if it is "drowned " by excess, the temperature 
is lowered, the process goes on slowly, and the mass does not 
readily fall into powder, and so fails to be utilised in the liming 
process. Of all methods of slaking lime, the ordinary one of 
tipping it direct into the lime-pits is perhaps the most irrational, 

^ Bull, de la Soc. d' Encouragement, 1895, x. pp. 52—62 ; Journ. Soc. 
Chem. Ind., 1895, P- 575- 



170 PRINCIPLES OF LEATHER MANUFACTURE 

leading to the formation of unslaked lumps which may burn the 
hides, and which, together with stones and dirt, rapidly choke 
the pits with useless matter. The best process is that adopted 
by builders and in many Continental yards, in which a large 
quantity of lime is slaked in a shallow tank by throwing on it 
sufficient water to thoroughly wet it, and after allowing it to 
heat and fall for twenty-four hours, adding enough water to 
convert it into a stiff paste. In this form it may be kept for 
months without material deterioration. When required for use, 
a suitable quantity of the paste is dug out, and well stirred with 
water in a tub or tank before running into the pit, when the stones 
and sand remain in the tank. In this way all nuisance from dust 
is also avoided. If lime is stored unslaked, it gradually absorbs 
moisture from the air, falling, and soon becoming dusty and 
difficult to slake completely, while the traces of carbon dioxide 
in the air gradually convert it into useless carbonate. 

The solubility of lime in water is very limited, and the figures 
determined by different chemists do not agree very satisfactorily. 
The following table gives the result of determinations made by 
Mr A. Guthrie in the Author's laboratory, and is probably one 
of the most accurate : ^ — 



100 c.c. of; 



t5°C.cc 


ntamo-i35ogrm 


10° 


0-1342 


15° , 


0-1320 


20° 


0-1293 


25° . 


0-1254 


30° , 


0-1219 


35^ . 


, o-ii6i 


40° , 


0-1119 


50° , 


, 0-0981 


6o° 


0-0879 


70° , 


0-0781 


8o° 


, 0-0740 


90° . 


0-0696 


00° 


0-0597 



of CaO. 



It will be noticed that unlike that of most substances, the 
solubility of lime in water diminishes as the temperature is 
raised. It is therefore necessary in employing lime-water as a 
standard solution to take care that it is saturated at a constant 
temperature. The results given in the above table are those 
from pure marble lime. Where the ordinary impure limes from 
limestone are employed, a somewhat stronger lime-water is often 



Journ. Soc. Cheni. Ind., 1901, p. 224. 



DEPILATION 



171 



obtained. This is difficult to explain, but possibly some double 
hydrate of lime and magnesia is formed which is more soluble 
than either hydrate alone. It is also possible that a lime may 
contain traces of soda or potash, or of baryta or strontia, which 
are more soluble than lime itself. These facts harmonise with 
the old belief of tanners that chalk-lime is milder in its action on 
skin than that made from less pure limestones. The solubility 
of any given lime is easily determined by adding it in excess to 
water in a stoppered flask, and shaking frequently until a solution 
of constant strength is obtained. A known volume of this 
solution (which must be clear and free from undissolved lime) 
is then titrated with N/io hydrochloric acid, using phenol- 
phthalein as the indicator. 

Saturated lime-water may be conveniently used as an alkaline 
standard solution for many purposes, and if kept on excess of 
lime, is always caustic, and varies very little in strength at ordinary 
laboratory temperatures. The solution is nearly 1/20 normal, 
but for accurate work its strength should be exactly determined 
with N/io acid. One liter of pure lime-water at 15° C. should 
require 471-4 c.c. of N/io acid for neutralisation. If a lime-water 
made by shaking good excess of lime with distilled water at 15° C. 
requires more than this amount of acid, it may be presumed that 
some other soluble base is present. Of course the amount found 
wiU in this case be dependent on the amount of lime used in excess. 

Lime is much more soluble in sugar solutions than in water. 
Such solutions have been used as standard solutions, and sugar 
has been added to limes to increase the action on the hides. 

The following is the analysis of a lime used in a Leeds tannery, 
which was made by Mr G. W. Flower, B.Sc, in the Leather 
Industries Laboratory at Leeds University : -^ — 

Per cent. 
SiO, and insoluble matter ..... I7"70 



FCgOg . 

CaO . 

CaCOg 

CaS04 

CaClg . 

MgO . 

Organic matter 

Moisture by difference 



6-42 
49-86 
14*21 

3-01 

0-33 
2-09 
o-8o 

5-58 



100-00 



The sample only contained 31-02 per cent, of available lime, 
the remainder being probably combined with the silica. It also 
^ Journ. Soc. Chem. Ind., 1901, p. 224. 



172 PRINCIPLES OF LEATHER MANUFACTURE 

contained an appreciable quantity of iron oxide, which might 
lodge mechanically in the pores of the skin and become dissolved 
in later processes, darkening the colour of the leather. The lime 
was also under-burnt, judging from the amount of carbonate it 
contained. 

For comparison with this, the analysis of a good specimen of 
carboniferous-limestone lime from Buxton may be given : — 

Per cent. 

CaO 91*95 

MgO 1-30 

CO2 and moisture 6-75 



100-00 



Determination of " Available " Lime. — The practical value of 
lime for the tanner is easily determined by drawing a sample by 
breaking off small pieces from a number of lumps of the bulk, 
coarsely pulverising them in a mortar, and then rapidly grinding 
a portion as fine as possible, and transferring it at once to a 
stoppered bottle for weighing. A portion of this, not exceeding 
I grm., is shaken into a stoppered liter flask, which is filled up 
roughly to the mark with hot and well-boiled distilled water, and 
allowed to stand for some hours with occasional shaking. When 
cold it is filled exactly to the mark with cold distiUed water, 
well shaken again and allowed to settle, or rapidly filtered, and 
25 or 50 c.c. of the clear liquid withdrawn with a pipette and 
titrated with N/io hydrochloric or sulphuric acid and phenol- 
phthalein. Each cubic centimetre of N/io acid equals -0028 
grm. CaO. If it be desired to determine separately the alkalies 
which may be present, standard N/io oxalic acid may be sub- 
stituted for hydrochloric, and after exact neutralisation a portion 
of the filtered solution may be acidified, and titrated with stan- 
dard permanganate for soluble oxalates, but it is extremely 
rare that alkalies are present in such quantity as to justify this 
trouble. The process is unfortunately not applicable to used 
lime-liquors, as they contain organic matter which would also 
reduce the permanganate. 

It is generally a very mistaken economy to make use of an 
inferior lime for tanning purposes, as any saving in cost is dis- 
counted by the larger quantity required, the more frequent 
cleaning of the pits, and the danger of stains and of burns from 
imperfect slaking. • 

The action of lime on the hide has already been spoken of to 
some extent. It is throughout a solvent one. The hardened 



DEPILATION 173 

cells of the epidermis swell up and soften, the mucous or growing 
layer and the hair-sheaths are loosened and dissolved, so that, on 
scraping with a blunt knife, both come away more or less com- 
pletely with the hair (constituting " scud " or " scurf," Ger. Gneist 
or Grund). The hair itself is very slightly altered, except at its 
soft and growing root -bulb, but the true skin is vigorously acted 
on. The fibres swell and absorb water, so that the hides become 
plump and swollen, and, at the same time, the " cement-sub- 
stance " of the fibres is dissolved, and they become split up into 
finer fibrils : the fibrils themselves become first swollen and 
transparent, and finally corroded, and even dissolved. A similar 
swelling of the fibres is produced by both alkalies and acids, and 
is due to weak combinations formed with *the fibre-substance, 
which have greater swelling power than the unaltered hide.^ This 
swelling is useful to the tanner, since it renders the hide easier 
to " flesh " [i.e. to free from the adhering flesh) on account of 
the greater firmness which it gives to the true skin. It also 
assists the tanning, by splitting up the fibre into its individual 
fibrils, and so exposing a greater surface to the action of the 
liquors. This is advantageous in dressing leather which is after- 
wards tanned in sweet liquors, and which must have the cement- 
substance of the fibres dissolved and removed for the sake of 
flexibility ; and, in the case of sole leather, it is necessary for 
sake of weight and firmness that the hide be plumped at some 
stage of the process ; but it is probable that this effect is pro- 
duced with less loss of substance and solidity by suitable acidity 
of the tanning-liquors. Another advantage of lime is that it acts 
on the fat of the hide, converting it more or less completely into 
an insoluble soap,- and so hindering its injurious effects on the 
after-tanning process and on the finished leather. If strong acids, 
whether mineral or organic, are used later on, this lime-soap 
is decomposed, and the grease is again set free. In sweated or 
very low-limed hides this grease is a formidable evil, causing 
darkening or grease spots on the finished leather. 

The customary method of liming is simply to lay the hides 
horizontally one at a time in milk of lime in large pits, taking care 
that each hide is completely immersed before the next is put 
into the pit, so as to ensure a sufficiency of liquor between 
them. Once a day at least the hides should be drawn out 
(" hauled "), the pit well plunged up to distribute the undis- 
solved lime through the liquor, and the hides then drawn in 
again (" set "), care being taken that they are fully spread 

1 Cp. Chapter X. 

^ This has been questioned, but I have satisfied myself it is correct. 



174 PRINCIPLES OF LEATHER MANUFACTURE 

'out. Neglect of this to save labour is very unwise. How 
much lime is required is doubtful, but owing to its Hmited 
solubility, an excess, if well slaked, is rather wasteful than 
injurious. Great differences exist in the quantity of the lime 
used, the time given, and the method of working, not only 
for various classes of leather, but for the same kinds in 
different yards. Lime, as we have seen, is only soluble to the 
extent of about 1-25 grm. per liter (as i cubic foot of water 
weighs about 1000 oz.), say i^ oz. per cubic foot, or, in an ordinary 
lime-pit, not more than J lb. per hide. Only the lime in solution 
acts on the hide, but it is necessary to provide a surplus of solid 
lime which dissolves as that in the liquor is consumed or absorbed 
by the hide ; and this is especially the case where, as is generally 
customary, the hides are laid fiat in pits, so that no circulation 
of liquor is possible. Where hides are suspended in lime-water, 
which is constantly circulated and kept up to its full strength 
by agitation with solid lime, they unhair more quickly than when 
laid in milk of hme, and the method seems to be gradually 
superseding the older one, especially in dealing with more 
soluble depilatories. Various patents have been taken for 
methods of liming by suspending in liquors, but the idea is now 
public property, and is largely used on the Continent. It is 
necessary that the Hme which settles to the bottom of the pit 
should be agitated and kept in suspension, which may be effected 
either by moving the hides on a frame as in " suspenders " 
(P- 356)' or by agitators acting on the principle of pumps, and 
raising the liquor and sludge from the bottom. Such agitators 
have been patented in Germany, but had been in use much 
earlier in the Author's tanyard. An agitator on the principle of 
the screw-propeller of a steamship, placed near the bottom of 
the pit, and protected by a lattice, has been much employed on 
the Continent (fig. 30). 

In England, suspension methods, though known for many 
years and in use in a few tanneries, received but little attention 
before the war ; but recently the saving of time, hide-substance, 
and labour which can be effected by suitable mechanical sus- 
pension and agitating appliances have led to their adoption in 
many large yards, often as more or less secret or patented pro- 
cesses, though it would be difficult to find a mechanical device 
which has not been used in times past. Such methods have the 
disadvantage of increased cost of plant, and the use of more pit 
space than is required by hides lying flat, though the latter is 
largely cancelled by the shortened time of the process and 
diminished quantity of lime required. The constantly increasing 



DEPILATION 175 

use of soluble depilatories, such as sodium sulphide, have also 







Fig. 30. — Suspension Lime-pit. 
increased their relative advantage. They are frequently com- 



176 PRINCIPLES OF LEATHER MANUFACTURE 

bined with means of mechanically handling the hides and re- 
moving them from pit to pit.^ 

Skins are frequently limed in paddles, or stirred up by blowing 
air into the pit. The latter method is neither effective nor 
economical in power for loose skins, but very convenient for 
suspended hides. 

As has been noted, the solubility of lime, and consequently 
the strength of the lime-liquor, is diminished by rise of tempera- 
ture, but its solvent action on hide-substance is much increased. 
As a consequence, the loosening of the hair proceeds much more 
rapidly in warm limes, but the hides do not plump well, and 
become loose, hollow, and inclined to " pipe " in the grain, and to 
" weigh out " badly, and for sole leather the method is therefore in 
every way disastrous. In the few cases among the lighter leathers, 
where a decided softening and loosening of the texture of the skin 
is required, it is possible that useful advantage may be taken of 
this effect ; but it would be exceedingly difficult to regulate the 
temperature of an ordinary lime-pit with accuracy, and better 
results could probably be obtained with suspenders m which the 
liquor could be constantly circulated. When limes are very 
cold, in spite of the greater strength of solution, the action is 
very much checked, and where goods are frozen into pits in 
severe weather, there is but little danger of over-liming, although 
the usual time may be much exceeded. It is generally best to 
work limes at about the ordinary summer temperature, and this 
is better done in winter by warming the lime-yard than by any 
direct heating of the limes. If lime which has cooled after slaking 
is used, the water with which limes are made may safely be 
warmed in midwinter to a temperature not exceeding 20° C. 

The quantity of lime used by different tanners, and for different 
sorts of hides and skins, is very variable, not only according to 
the effect which it is desired to produce, and the way in which 
it is used, but from the arbitrary fancy of the user, since its 
limited solubility renders an excess comparatively innocuous. 
For sole leather, the amount recommended varies from under 
I per cent, to 10 or 12 per cent, on the green weight of the hide ; 
but probably 2 to 3 per cent, is all that can be really utilised, the 
remainder being wasted. In order, however, to utilise the whole 
of the lime, very frequent handhng or agitation is required to 
ensure its uniform distribution. It must also be borne in mind 
that the strength of commercial limes varies from above 80 down 
to 30 per cent, of available calcium oxide. 

Von Schroeder has found that a strength of 6 grms. of calcium 
1 See Eng. Pats. 117581 and 124992. 



DEPILATION 177 

oxide (CaO) per liter was sufficient, but, in practice, much more 
is generally added. It is also noteworthy that a perfectly fresh 
milk of lime is much less rapid in action than one which has 
been used. This is partially due to the fact that some bacterial 
action takes place in an old lime and that ammonia is formed 
which assists unhairing, in addition to the effect of the lime 
itself, and partially because the lime in old liquors remains in 
suspension for a much longer time, and is thus more evenly 
distributed. 

A method of liming, sometimes known as the " Buffalo method," 
has been largely adopted for sole leather in America, and is now 
used in many Continental yards. It consists in a very short 
liming and the subsequent use of warm water. The limes are 
also often sharpened by the addition of a little sodium sulphide 
or of some other sulphide. Thus, in one large yard in the States, 
the hides for sole leather (salted " packers ") are limed for ten 
hours only with 2 lb. lime and 2| oz. of sulphide of sodium per 
side, and after lying overnight in water of a temperature of 
35° to 45° C. are easily unhaired. A Continental firm lime two 
days in weak fresh limes with a little tank-waste, and then treat 
with water at 32° C. for six to eight hours, when the hides are 
unhaired and returned to the warm water for two hours before 
scudding. All sorts of combinations between liming and hot- 
water treatment can be employed. The longer and stronger the 
liming, the lower temperature or shorter time in the water will 
suffice. The method is much to be recommended for firm sole 
leather, but it does not saponify grease or swell the fibres 
thoroughly, and usually sulphuric acid is used for the latter 
purpose in a later stage. The hide goes into the liquors prac- 
tically free of lime, and the loss of hide-substance is much 
less than in the ordinary method of liming. 

A point of probably much greater importance than the quantity 
of lime used is the length of time during which a lime is worked 
without change of liquor. An old lime becomes charged with 
ammonia and other products of the action of lime upon the skin, 
such as tyrosin, leucin (aminocaproic acid), and some caproic 
acid, the disagreeable goaty odour of which is very obvious on 
acidifying an old lime-liquor with sulphuric acid, by which con- 
siderable quantities of partially altered keratins are at the same 
time precipitated {cp. p. 134). A test supposed to indicate dis- 
solved hide-substance in old limes has been employed in some 
yards, in which the liquor is slightly acidified with acetic acid, 
and an equal volume of saturated salt solution is added, when, 
on standing, a precipitate rises to the surface, the volume of 

12 



178 PRINCIPLES OF LEATHER MANUFACTURE 

which is supposed to indicate the dissolved hide-substance. 
(Cp. L.C.P.B., p. 41.) It has been recently shown in the 
Procter International Research Laboratory that actual dissolved 
hide-substance is not precipitated under these conditions, and 
that the precipitate formed is entirely derived from the 
albuminoids (keratins) of the hair and epidermis, which it is 
the definite object of liming to dissolve.^ 

Lime has considerable antiseptic power, and a new lime is 
practically sterile, but very old limes, especially in hot weather, 
often contain large numbers of active bacteria, which may be 
seen in the microscope under a good ^-inch objective. Their 
presence is always an indication that putrefaction is going for- 
ward, and if their number be very excessive, the leather out of 
such limes will generally prove loose, hollow, and dull-grained, 
and in extreme cases hides may be totally destroyed. Spherical 
concretions of calcium carbonate may also be seen under the 
microscope, resembling on a smaller scale those found in Permian 
limestone, and caused perhaps in both cases by crystallisation 
from a liquid containing much organic matter. It is hardly 
probable that in many tanneries the ammonia would pay for 
recovery from the lime-liquors, though it could be easily done by 
steaming the old limes in suitable vessels, and condensing the 
ammoniacal vapours in dilute sulphuric acid. Its quantity rarely 
exceeds o-i per cent, of NH3. For methods of estimation of 
ammonia see L.I.L.B., p. 30, and L.C.P.B., p. 39. 

Up to a certain point it is found that old limes unhair much 
more readily, and have a greater softening effect than new ones, 
which is often advantageous for dressing goods ; though for sole 
leather, where weight and firmness are of primary importance, 
the use of stale limes must be kept within the narrowest limits. 
In the finer leathers also, such as kid and moroccos and coloured - 
calf, where a sound and glossy grain is desired, the effects men- 
tioned are generally better obtained in other ways, such as by 
the use of sulphides. On East India kips and other dried hides, 
which are difficult to soften, and which have great power of 
resistance to the action of lime, old limes are distinctly useful, 
but, even there, there are limits which should not be passed, 
and the tendency of modern practice is against their use. 

Probably no lime ought to be allowed to go for more than 
three months at the outside limit without a change of liquor, 
and the system of allowing all the limes in a yard to run for 
twelve months, and then cleaning them all together, is almost 
the worst which can be planned. A very much better way is to 
1 Thompson and Atkin, J.S.L.T.C., 1920, p. ij. 



DEPILATION 179 

clean the limes in regular rotation, using, if desired, a portion of 
the old liquor in making the new lime, so as to avoid a too sudden 
transition. The old liquor is valuable, if at all, for the ammonia 
and organic matter which it contains, as the amount of lime in 
solution is not worth considering. The ammonia considerably 
increases the solvent and unhairing power, while swelling the 
hide less than an equivalent amount of lime. In some cases it 
may be desirable to add ammonia artificially for this purpose. 
In this case it will be cheaper and more convenient to add it in 
the form of ammonium sulphate than as liquid ammonia. If it 
be desired to retain ammonia, the lime should be kept covered. 
Very old limes containing excess of ammonia and lime some- 
times in hot weather cause a transparent swelling of the goods, 
with destruction of the fibrous texture. ■•■ This condition is not 
uncommon in sheep-skins which have been over-limed in fell- 
mongering, or in order to make them sufficiently firm for splitting, 
and is known as " greenstiffness." Such skins come down with 
difficulty or not at all in the puers, and never make satisfactory 
leather. The writer has observed a similar phenomenon in very 
weak and old limes strengthened with sulphides, in which hide 
was left experimentally for several weeks. This effect is not fully 
understood, and deserves further investigation. The principal 
effect of the dissolved animal matter is to enable bacteria to 
thrive in it, which they will not do in a fresh lime, but putrid 
limes also contain liquefying ferments produced by the bacteria 
present (p. 19), which dissolve hide. Eitner published researches 
on the amount of hide-substance dissolved by limes, in which he 
shows that the loss of substance in liming sufficiently to unhair is 
materially greater in old limes than in fresh ones, although during 
the first two days of liming the new limes are decidedly the most 
active. As he remarks, this justifies the wisdom of the method, 
now largely adopted, of working limes in shifts, and beginning 
the operation in old limes and completing it in fresh ones. (See 
also p. 180.) He also proves that the loss in limes sharpened 
with sulphides is less than in those made with lime alone, and that 
they deteriorate more slowly. His methods of analysis are now 
somewhat antiquated, and do not differentiate between real loss 
of hide-substance and mere solution of epidermal matter, so 
that it does not seem worth while to reprint in detail, for which 
the reader must be referred to the original papers, ^ but he shows 
the very considerable loss which may occur in plumping limes 
after the hair has been removed. 

1 Gerber, 1884, pp. 150, 184. 

^ Ibid., 1895, pp. 157-9, 169-72. 



i8o PRINCIPLES OF LEATHER MANUFACTURE 

The parts taken by the purely chemical activity of the lime, 
and by the action of bacteria and bacterial ferments in the un- 
hairing process, must still be regarded as uncertain. The late 
Professor von Schroeder^ carried out a series of experiments on 
liming and sweating which were characterised by his usual care 
and thoroughness, and which tend to prove that the chemical 
action is far more important than the bacterial. He had fresh 
hides well washed in a tannery immediately after slaughter, and 
fleshed. The butts were then cut into pieces of about lo cm. 
(4 inches) square, and salted in brine repeatedly changed, and 
finally preserved for use in glass jars in saturated salt solution. 
He found that when washed free from salt, and placed in a moist 
chamber at a temperature of 16° C, the hair was sufficiently 
loosened by bacterial action in four to five days. Pieces placed 
in the moist chamber without previous removal of the salt only 
showed signs of sweating after about ten weeks' exposure. 
Liming experiments were made with similar pieces of salted hide, 
both after three days' washing to free them from salt, and un- 
washed, and in both cases the pieces unhaired freely in three 
to four days. These experiments were varied by using 6, 18, and 
30 grms. of lime per litre of water in which about 200 grms. of 
hide were placed, but neither in the washed nor unwashed portions 
was there any material difference in the time required to loosen 
the hair. Addition of i vol. of used lime-liquor to 3 vols, of 
water in making up the limes was equally without perceptible 
influence, and careful bacteriological examination of hide and 
liquors showed that the former was almost sterilised by the 
intense salting, and that the lime-liquors were practically free 
from bacteria. 

Von Schroeder's conclusion that no gain arises from the use 
of excessive quantities of lime, so long as the solution is kept 
saturated, is fully justified both by experience and scientific 
reasoning, but his results with regard to the effect of old liquors 
and bacteria contradict the conclusions both of practical tanners 
and of other scientific experimenters. 

The different effects of old and new limes are too well known 
to practical tanners to be discounted by laboratory experiments, 
even if they were not confirmed not only by Eitner's results, but 
by a considerable amount of work done in the Author's labora- 
tory and elsewhere ; while the necessity of bacterial action is at 
least rendered probable by the fact that soda solutions which are 
completely sterile to bacteria, fail to unhair hides which have not 
previously undergone some putrefaction (see p. 183). In some 
^ Gerberei-Chemie, Berlin, 1898, p. 646. 



DEPILATION i8i 

experiments undertaken at the suggestion of the Author it was 
found that a perfectly fresh and steriHsed calf-skin which was not 
unhaired after ten days' liming in sterilised lime-liquor unhaired 
rapidly on the addition of a bacterial culture to the lime. It is 
extremely difficult to exclude bacteria, and even where perfectly 
fresh skins treated with chloroform or carbon disulphide were 
employed, bacteria were always to be recognised when the skin 
was ready for unhairing. Von Schroeder's work was, however, so 
painstaking and reliable, that these divergent results must be 
explained as other than experimental errors. With regard to 
old liquors, it is known that ammonia is a powerful aid to the 
unhairing process, and it is not certain to what extent the liquors 
he used were charged with it. It is also certain that old limes 
containing much organic matter support bacterial life freely, 
while 25 per cent, of a possibly not very old liquor would probably 
be sterilised by the addition of lime and 75 per cent, water. In 
order to test the matter fairly under, exact tannery conditions, 
the lime should have been made up entirely with old lime-liquor 
well charged with, ammonia and organic matters instead of with 
water. It is also probable that the hides had undergone a suffi- 
cient amount of bacterial change in the tannery before they came 
into Von Schroeder's salt solutions, and it is not at all unlikely 
that the salt solution itself exercised some specific effect on the 
unhairing. It is also possible that his bacterial cultures were 
made on gelatin media unsuitable for the growth of alkaline 
bacteria, and therefore gave blank results. Under these circum- 
stances it is scarcely possible to arrive at any very definite con- 
clusions, and it is obvious that further experiments on these 
points are extremely desirable. 

Sodium and Potassium Hydrates. — From the earliest antiquity, 
wood-ashes, consisting mainly of potassium carbonate, have been 
used for unhairing, either alone or in conjunction with lime, 
and indeed the German name of the process {Aeschern) is derived 
from the fact. In more recent times, caustic soda, either ready 
formed or causticised on the spot by the addition of lime, has 
often been recommended as a substitute for lime. Its action is 
very similar to lime, but, from its greater solubility, is far more 
powerful, and probably this has hitherto formed one of the 
greatest obstacles to its use, since a solution of the strength of 
lime-water is almost immediately exhausted, while a much 
stronger one is too violent in its action on the hides. Some 
experiments made in the Author's laboratory appear to show that 
caustic soda, in solutions of the same strength as lime-water, 
dissolve considerably less hide-substance than the latter, but it is 



i82 PRINCIPLES OF LEATHER MANUFACTURE 

more antiseptic than lime, and does not unhair readily without 
the aid of bacterial action {cp. p. 183). It also swells more 
violently, and it is difficult to keep the grain smooth and 
unwrinkled, and, from its rapid action and the necessary 
dilution of the solution used, should only be employed in 
suspension-limes, or other appliances for rapid circulation of 
liquors. 

Wilson points out ^ that the compounds of monovalent bases, 
such as soda, potash, and ammonia, are likely to be more ionisable 
and more soluble than those of divalent bases such as lime and 
baryta, and therefore to produce greater swelling and more 
solution of hide-substance, and this view is in accordance with 
known facts. J. Loeb has recently shown {loc. cit.) that the swell- 
ing power of acids and bases is inversely as the valency of the 
anions or kations respectively. For example, in solutions of 
equal hydroxyl-ion concentration, caustic soda exerts twice the 
swelling power of lime, since Na' is a monovalent, and Ca" a 
divalent kation. Similarly hydrochloric acid has twice the effect 
of sulphuric acid in solutions of equal hydrion concentration. 
This work no doubt applies directly to hide and other proteins. 
Ammonia, it is true, swells less than lime because of its " weak- 
ness " as a base, which renders the actual concentration of 
hydroxyl-ions in an ammonia solution much less than in one of 
lime of equivalent concentration, but, notwithstanding this, it 
dissolves more hide-substance. 

Caustic soda has the great advantage that from its solubility, 
and that of its carbonates in water, it is much more easily and 
completely removed by washing after neutralisation than is the 
case with lime. It has been successfully applied in some instances 
to soften skins of which the texture is naturally too compact for 
moroccos and the softer leathers, and is usefully employed in 
softening dried goods (p. 162). Where caustic soda is required 
merely to " sharpen " limes, it is best added in the form of 
sodium carbonate (soda-ash or crystals), which are causticised by 
the lime in the pits. One quarter or one-half per cent, on the 
weight of hides added in this way decidedly increases the plump- 
ing power of the lime. Some natural waters in Yorkshire con- 
tain sufficient sodium carbonate to produce this effect. It may 
be noted that in the use of sodium sulphide in conjunction with 
lime caustic soda is one of the products of its decomposi- 
tion, and is one great cause of the difference of effect of this 

1 " Theories of Leather Chemistry," J.A.L.C.A., 1917, p. 112. See also, 
on this and many other important points, particularly sharpening of limes, 
E. Stiasny, Gerber, 1906, translated in J.S.L.T.C., 1919, 4, p. 129. 



DEPILATION 183 

material for sharpening limes as compared with red arsenic 
[cp. p. 189). 

An indirect method of liming was patented by Messrs Payne 
and Pullman of Godalming/ which is of both scientific and 
practical interest. From the difficult solubility of lime, and 
the consequently weak solutions which must be employed, the 
ordinary process of liming is a slow one. Caustic soda, however, 
can be used in much stronger solutions without producing injury 
to the hide, or larger solution of hide-substance, as, from its great 
diffusibility, it penetrates very rapidly. Used alone, however, 
the hide becomes too much swollen for most purposes, and for 
certain classes of leather at least {e.g. buff and chamois leather) 
the presence of a portion of lime in the hide appears to be 
necessary for successful work. If a hide which has been swollen 
with caustic soda be afterwards treated with a solution of calcium 
chloride, double decomposition takes place, and caustic lime is 
formed actually in the interior of the fibre of the hide, while the 
sodium unites with the chlorine to form common salt. Both 
solutions may be used in any convenient way, and by the employ- 
ment of drums the whole liming process may be accomplished 
in five or six hours. It was found, however, that perfectly fresh 
hides treated in this way could not be unhaired, and the explana- 
tion appears to be that in the ordinary liming process the 
epidermis is made soluble by the joint action of bacterial ferments 
and of the alkaline solutions. If sodium sulphide be added to 
the caustic soda used for unhairing the goods will unhair without 
the use of putrefactive means, but the process is difficult to 
manage without destruction of the hair, and Messrs Pullman later 
recommended that all hides or skins for unhairing by their process 
should be soaked for forty-eight hours in winter and twenty-four 
hours in summer in a really putrid stale soak. This necessity 
constitutes for very many purposes a serioiis weakness in the 
method, as putrid soaking is always extremely dangerous to the 
grain of the hide, and especially so in hot weather. For certain 
purposes, however, advantage may be taken of the fact that the 
hide or skin can be fully limed by Pullmans' process and the 
fibres swollen so as to be prepared for tanning without any 
loosening of the hair, and on fresh skins which have been 
treated in this way the hair remains perfectly firm, while they 
possess a softness and fulness which could not be attained without 
liming. 

Messrs Pullman later recommended that the treatment with 
their solutions should take place in pits, in preference to drums or 
1 Eng. Pat. 2873, 1898. E. M. Payne, J. & E. Pullman. 



i84 PRINCIPLES OF LEATHER MANUFACTURE 

paddles, and that the caustic soda should not exceed a strength 
of I lb. in 10 gallons (i per cent.). The hides or calf -skins 
remain in this for about forty-eight hours, during which they are 
once drawn and returned, by which time, if the putrid soaking 
has been properly done, the hair should be fully loosened. The 
hides are then drained for two hours, and passed into another 
pit containing a solution of calcium chloride, which should be 
slightly stronger than the caustic soda, say, of about i| lb. 
per 10 gallons. The goods remain in this for about forty-eight 
hours, during which they are drawn once, and are then well 
washed in soft water (free from temporary hardness), in which 
they may be kept for some time without injury. As the caustic 
soda and the calcium chloride solutions are quite sterile to 
ordinary putrefactive bacteria, both can be used for an almost 
unlimited time, and they are conveniently kept up to strength 
by the addition of strong stock-solutions. These may be 
made of a sp. gr. of 1-4 (80° Tw.), which gives a strength of 
about 5j lb. of caustic soda and 5f lb. of calcium chloride per 
gallon. 

In addition to the advantage of considerable saving of time, 
the effects can be much more easily regulated than in ordinary 
liming, and the amount of soda (and subsequently of lime) 
absorbed by the hide can be exactly determined by titration of 
the liquors. Grease is better removed than by ordinary liming, 
as soda-soaps are soluble in water, but if this result is to be 
obtained, the soap must be worked out before passing into the 
calcium chloride solution, which would otherwise convert it into 
an insoluble lime-soap. A great gain in many districts is that 
the process yields practically no effluents and no lime ■ slab, 
both of which are frequently very difficult to dispose of. The 
serious disadvantages of the stale soaking, however, have 
already been mentioned, and the method has not come much 
into use. 

In place of applying the caustic soda first and the calcium 
chloride subsequently, hides may be first treated with calcium 
chloride solution and then with caustic soda, or the caustic soda 
may be applied to the flesh side of the hide by painting. 

Alkaline carbonates are much milder in their action on hide 
than the corresponding hydrates, as they owe their effect to the 
small quantity of hydrate formed in their solutions by hydrolysis. 
As this is an equilibrium-reaction, and the hydrate is only repro- 
duced as it is consumed, the effect is automatically self-regulating, 
like that of the hmited solubihty of hme. The unhairing action 
is somewhat slow and the swelling moderate, and they deserve 



DEPILATION 185 

more attention, but the best conditions for their use have not 
been sufficiently studied. In presence of Hme they become 
causticised, and act like an addition of NaOH. 

Sodium carbonate occurs in commerce in three forms : " soda 
ash," a more or less pure dry sodium carbonate ; " soda crystals," 
or washing soda, NagCOg . loAq, containing 62-95 per cent, of 
water of crystallisation, and efflorescing in the air ; and Gaskell 
and Deacon's " crystal soda," NagCOg . lAq, containing only 
14-5 per cent, of water of crystallisation. 

Sulphides. — The practice of using realgar, or red sulphide of 
arsenic (Ger. Rusma), as an addition to limes for fine leathers is 
one of considerable antiquity. It has the property of loosening 
the hair and epidermis structures with less solution of cement- 
substance than lime alone, and hence produces a leather of fuller 
and closer texture. It will, however, be convenient to defer the 
consideration of this agent till after that of some of the more 
modern and simpler substitutes, such as the sulphides of sodium 
and calcium. Sulphides of the alkalies and alkaline earths, if 
used in strong solution, say 5 per cent, or upwards, have the 
effect of very rapidly reducing the harder keratin-structures, 
such as hair and wool, to a pulp, attacking first the interior cells, 
so that each hair crumples up like a string of sausages, and in a 
few hours, or even, with very strong solution, in a few minutes, 
the whole mass becomes so completely disintegrated than it can 
be swept off the hide with a broom, or washed off in a tumbler. 
At the same time, the action on the substance of the hide, and 
especially on the cementing substance, is very slight, though the 
grain is swollen and temporarily rendered somewhat tender. On 
the other hand, when used in weak solutions, say J per cent, and 
under, in conjunction with lime, the hair is but little injured, 
while the hair-roots and dirt are rapidly loosened, and results 
are obtained very similar to those with red arsenic. 

Sodium stdphide (Na2S,90H2).-^ — For the methods of valua- 
tion and determination of sodium sulphide, see L.I.L.B., p. 28, 
and L.C.P.B., p. 32. 

It is very soluble in water, of which it combines with i mol., 
forming equal amounts of NaSH and NaOH. 

Hides suspended in solutions of sulphide of sodium of 2 to 3 
per cent, strength unhair rapidly. 

For the commoner classes of sole leather, hair is frequently 
removed by painting on the hair side with a 15° to 28° Tw. (30 

1 In the Laboratory Book the water of crystallisation is given as 10 Aq. 
Later researches show that pure crystals of the commercial sulphide only 
contain 9 Aq, or 67-5 per cent, of water. 



i86 PRINCIPLES OF LEATHER MANUFACTURE 

to 40 per cent.) solution of (crystallised) sulphide of sodium 
thickened with lime, applied with a fibre-brush, and folding the 
hide in cushions in a damp place, or packing in a tub. The hair 
is reduced to paste in a few hours. The same effect is produced 
by drawing the hides through a similar solution without lime, of 
which sufficient is retained by the hair to destroy it. The work- 
men must be provided with indiarubber gloves to prevent the 
caustic effect of the solution on the skin and nails. Skins and 
lighter hides are conveniently unhaired by painting the mixture 
on the flesh side, when it will loosen the hair or wool in a few hours 
without destroying it. 

For dressing leathers and the finer sorts of sole sodium sulphide 
is best employed as an addition to ordinary limes to the extent 
of J to I per cent, on the weight of the hides or skins, when the 
hair is loosened more rapidly than with lime alone, and with less 
loss of hide-substance. 

Mixed with water to the extent of 2 to 3 lb. per gallon, and 
thickened with lime to a soupy consistence, it is much used for 
unwoolling sheep-skins. The wet skin is laid flesh up and painted 
with the mixture, care being taken that none of it touches the 
wool, which it would destroy. The skins are doubled down the 
back, and laid on loose boards so as to allow of drainage, the back 
of each skin being placed on the flank of the preceding, like the 
slates on a roof, and after lying overnight the wool is readily 
pulled, and the pelts receive a further liming to loosen the wool 
on the edges and complete the process. 

It has been suggested by the Author that calf and other skins, 
which when chromed often prove loose on the flanks, might 
profitably be painted on the thicker parts on the flesh, possibly 
with a weaker solution than that just given, and after lying a 
short time, might be limed in the ordinary way to an extent only 
sufficient to loosen the hair on the thinner parts, so leaving them 
in a firmer condition. 

For some classes of skins the powerful swelling effect of the 
sodium hydrate formed by reaction with water, and the further 
quantity by reaction of the sodium sulphydrate with the lime.i 
is objectionable, as causing too much swelling, and the use of 
arsenic sulphide, which does not produce sodium hydrate, is 
preferred. The difficulty can, however, to a large extent be 

1 Professor von Schroeder expresses the opinion that lime and sodium 
sulphydrate do not react in this way, but, according to modern views, it 
is obviously an equilibrium-reaction, depending on the concentrations of 
the ionised portions of both, and at least some proportion must exist in 
solution, if not in a solid form. 



DEPILATION 187 

overcome by adding an equivalent of calcium chloride, which 
reacts with the sodium hydrate, forming common salt and calcium 
hydrate. It is often said that sodium sulphide does not swell, and 
this is true when used for painting in strong solution on account 
of the repressive action of the concentrated solution, but if the 
goods are placed in water great swelling takes place, so much 
so that if allowed to soak too long they may become too swollen 
for convenient fleshing ; and there is no need for the subsequent 
swelling with sulphuric acid which is sometimes given, though 
neutralisation with this or some other acid is desirable, since, 
though all the products of the reaction are quite soluble, the 
compound of alkalies with hide is only very slowly decomposed 
by water alone. During the neutralisation a considerable amount 
of hydrogen sulphide is evolved. 

Good samples of sulphide of sodium consist of pale brown, 
almost colourless crystals, containing 28 to 32 per cent, of dry 
sodium sulphide, which readily deliquesce on exposure to air. 
Fused sodium sulphide can now be obtained which contains 
nearly twice as much actual sulphide as the crystalline form. 
The dark green colour possessed by many samples of sodium 
sulphide is due to the presence of iron sulphide. If carefully 
used no serious harm can accrue from its presence. If allowed 
to stand a short time in solution the iron sulphide will settle 
out. 

Calcium sulphydrate, Ca(SH)2, sometimes called Bottger's 
Griinkalk, is a powerful depilatory, which has even less destruc- 
tive action on the hide-fibre than sulphide of sodium, and 
would no doubt be largely Used but for its unstable character. 
It is probably the principal active product produced by the 
use of sulphide of arsenic in conjunction with lime, though 
possibly a sulpharsenite may also be formed. It may be pro- 
duced by passing hydrogen sulphide (SHg) into milk of lime, or 
byihe reaction of sodium sulphide on lime solutions. It may be 
obtained crystallised, and is soluble in water, but is decomposed 
on boiling, by the carbonic acid of the air, and by oxidation. The 
sulphide, CaS, is insoluble in water, but by the action of steam 
under pressure it is said to be converted into a mixture of equiva- 
lent parts of hydrate and sulphydrate. It may also be dissolved 
in a solution of hydrogen sulphide, forming a solution of 
sulphydrate. In this way it might be produced on a large scale 
from the " tank- waste " of the Leblanc soda process. 

Gas-lime is principally active on account of the calcium sul- 
phide which it contains, but is very variable in its strength, as 
both sulphydrate and sulphide are decomposed by the carbon 



i88 PRINCIPLES OF LEATHER MANUFACTURE 

dioxide always • present in the gas, forming carbonates. Lime 
has nearly gone out of use for purifying gas, its place being now 
taken by iron oxide, but formerly gas-lime was a good deal 
used for unwooling the small lamb-skins used for the commoner 
sort of glove-kid, usually by painting a cream of it on the flesh 
side, but sometimes by immersing in a strong solution, which of 
course destroyed the wool. Its place is now taken by a solution 
of sodium sulphide of 15° to 18° Tw. (approximately 30 to 35 per 
cent, crystals), thickened with lime to a soupy consistence, the 
use of which is much to be recommended for unwooling sheep- 
skins, by painting on the flesh. 

The tank-waste from the Leblanc process, consisting princi- 
pally of calcium sulphide, is, when fresh, quite insoluble, and has 
no depilatory powers ; but when exposed to air and moisture 
decompositions take place, resulting in the formation of sulphy- 
drates and polysulphides, which form a solution which has been 
the subject of several patents for unhairing.^ Polysulphides 
alone have probably no unhairing effect, but in conjunction with 
lime, sulphydrates are formed which rapidly loosen the hair. 
This fact was the basis of an ingenious and effective unhairing 
process used very many years ago by Mr John Muir of Beith, 
who, after liming for twenty-four hours in the usual way, sub- 
mitted the hides to a pretty strong solution of weathered tank- 
waste for twenty-four hours, and finally to water for twenty-four 
hours, to remove the surplus lime and sulphides. The sulphy- 
drates formed in the hide attacked the hair-roots with little 
injury to the hair itself, and the hides contained so httle lime that 
they could be tanned for dressing without bating, and made 
about 10 per cent, more weight than those treated in the ordinary 
way. Some trouble was occasioned by stains caused by impurities 
in the tank-waste. 

A somewhat similar unhairing mixture to that obtained from 
tank-waste, which is now seldom to be got since the introduction 
of the ammonia-soda process, was patented by Professor Lufkin,^ 
who mixed equal parts of sulphur and soda-ash with a little 
water till combined, and then added 8 to 10 parts of lime, slaked 
and still hot. Schultz^ states that such a mixture containing 
10 lb. of sulphur will unhair fifty hides in the same way, and in 
about the same time as an ordinary lime, the pelt being little 
plumped and easily reduced without bating by a few minutes' 
wheeling in warm water. By boihng lime and sulphur with 
water a yellow solution is obtained which can be used in the 

1 Squire, Eng. Pat. 75O, 1855; Claua, Eng. Pat. 1906, 1855. 

- Eng. Pat. 2053, i860. « Leather Manufacture, p. 35. 



DEPILATION 189 

same way as that from the tank-waste. A further quantity of 
water can be boiled on the same materials, more lime and sulphur 
being added as required. Polysulphides appear to have a marked 
effect in preventing or reducing alkaline plumping, and apparently 
could be used in some cases with advantage as bates. On acidi- 
fication they give off SHg and precipitate sulphur, so that if used 
for neutralising one-bath chrome leather they deposit sulphur 
on and between the fibres, and imitate that of the two-bath 
process, where sulphur is deposited by the decomposition of the 
thiosulphate. Sodium or potassium polysulphide can also be 
used for reducing in the two-bath process in place of thiosulphate. 
Sodium polysulphide is produced by boiling sodium hydrate, 
carbonate, or sulphide with excess of sulphur. It is a clear yellow 
solution, with a smell of hydrogen sulphide.^ 

Barium sulphydrate has been put on the market experimentally 
as an unh airing agent in the form of a strong solution containing 
yellow polysulphides, and which deposits crystals of sulphydrate 
in cold weather. It is more stable than calcium sulphydrate, 
but, on the whole, does not seem to present any great advantage 
over sodium sulphide, though it is quite effective as a depilitant. 

Realgar or red sulphide of arsenic, AsgSg, is made by. fusing 
arsenious acid and sulphur, and is of somewhat indefinite com- 
position. Mixed with lime it produces calcium sulphydrate, and 
possibly hyposulpharsenite. To produce a rapid and complete 
reaction it must be mixed with hot lime, and the hotter the 
mixture is made the more powerful is its unhairing action. 
Milder forms may be made by mixing cold, or with the aid of hot 
water only. It is used with great advantage in conjunction with 
lime in varying proportions for unhairing lamb- and kid-skins 
for glove-kid and other fine leathers, to which it gives the necessary 
stretch and softness and cleanness of grain, without the loosening 
of texture and loss of hide-substance which would be caused by 
an equivalent amount of ordinary liming. For glove-kid about 
o-i to 0-3 per cent, of realgar and 5 per cent, of lime is used, 
reckoned on the green weight of the skin. The mixture is added 
to the ordinary limes, according to the judgment of the tanner 
and the character of the skins. 

For painting the flesh side of calf- and lamb-skins one part of 
realgar is mixed with ten parts of hot lime, made into a paste 
with water. Calf will unhair in eight or ten hours. 

Orpiment (auripigmentum) , ASgSg, is the yellow precipitate pro- 
duced in arsenical solutions by sulphuretted hydrogen, and is a 
by-product of the de-arsenification of sulphuric acid made from 
1 See J. E. Pickles, J. S.C.I. , 1916, p. 456. 



190 PRINCIPLES OF LEATHER MANUFACTURE 

arsenical pyrites. It is consequently generally cheaper than 
realgar, and experiments in the laboratory of the Leeds Leather 
Department showed it to be more powerful as a depilatory, as is 
to be expected from its larger proportion of sulphur, but tanners 
for some reason, or very probably from prejudice, have generally 
refused to use it.^ 

An unhairing solution has been sold under the name of 
" Inoffensive " which contained a large quantity of arsenic 
sulphide apparently dissolved in caustic soda, although Moret's 
original patent claimed the use of wool-sweat potash only ! 

W. R. Earp ^ has suggested the use of compounds of sulphur 
and arsenic (thioarsenates, thioarsenites, etc.) in 5 per cent, 
alkaline solution. He prefers to add the compounds to the 
ordinary lime-liquors, or to manufacture them in situ by adding 
the proper quantities of arsenious or arsenic acid mixed with 
one-third of its weight of sulphur to a solution of an alkaline 
sulphide in lime-liquor. The pelt is not bated or drenched in 
the ordinary way, but, after unhairing, is passed directly into 
the tanning liquor, to which sulphurous acid has been previously 
added. 

There is more danger of injury to the hide from the very 
prolonged action of weak solutions of sulphides, which tend 
ultimately to destroy the structure and reduce the fibre to a 
gelatinous condition, than there is from too concentrated solutions. 
No danger need, however, be apprehended in the course of any 
ordinary liming. Arsenical limes are not suited for tainted skins, 
and they should not be made so strong as to destroy the hair 
or wool. 

All these arsenical compounds are very poisonous, and should 
go out of use for depilation, as they do not appear to be necessary. 
Alkaline solutions of arsenious acid are, however, useful in pre-, 
venting insect-damage to dry hides, though they have little or 
no antiseptic effect, either for that purpose or in limes (see 
p. 42). 

Dr Rohm, the inventor of " oropon " (see p. 220), has recently 
patented the use of the tryptic enzyme for unhairing as well as 
for puering. There is no reason that it should not do this quite 
effectively, as it digests epidermis matter without attacking the 
fibre of the hide, but it has not been sufficiently tried yet to 
enable us to pronounce on its commercial value. ^ 

For the analysis of both old and new lime-liquors see L.I.L.B., 

^ See, on the action of arsenic sulphides, E. Stiasny, loc. cit. 

2 Eng. Pat. 2052. Feb. 12, 1886. 

3 D.R.P., 268, 873. See also Hollander, J.A.L.C.A.. 1920, p. 477. 



DEPILATION 



191 



pp. 27-34, and L.C.P.B., pp. 31-41. It must be admitted, how- 
ever, that the present control-methods leave much to be desired in 
the determination of dissolved hide-substance and the distinction 
between this and the dissolved epidermis, but researches are in 
progress in the Leather Industries Department and the Procter 
Research Laboratory which promise a great improvement in this 
respect.^ 

Whichever method of loosening the hair be adopted, the actual 
removal must be effected by mechanical means, and is still very 




Fig. 31. — Unhairing. 

usually accomplished by placing the hide on a sloping beam with 
a convex surface, and then scraping it with a blunt two-handled 
knife (fig. 31), the workman pushing the hair downward and away 
from himself. The beam may be either of cast iron or of wood 
usually covered with zinc to increase its wearing capacity. The 
hides after being removed from the lime-pits are allowed to drain 
for half an hour or so before the hair is removed, and immediately 
this operation has been completed they should be placed in soft 
water. It is of great importance that the limed hides should not 
be exposed to the air longer than is absolutely necessary for the 
removal of the hair, as the carbonic acid present in the atmo- 



^ See four papers on Limeyard Control in J.S.L.T.C., 1920, also 
Appendix. 



192 PRINCIPLES OF LEATHER MANUFACTURE 

sphere quickly carbonates any lime contained in the surface of 
the skin, forming chalk, and leading to uneven tanning at a 
later stage. 

When hide has been insufficiently limed it is often easy to 
remove the longer hair but excessively difficult to get rid of the 
short under-growth of the young hairs, which even in properly 
limed skins can sometimes only be removed by shaving them with 
a sharp handknife. This difficulty is caused partly by the small 
resistance which the short hairs offer to the unhairing-knife, and 
partly by their being more deeply rooted in the skin than the 
older hairs (see p. 55). 

Various machines have been devised to accomplish the removal 
of the hair, but owing to the rapidity with which it may be 
worked off by hand, and the fact that the work is not difficult, 
they have hardly as yet come into general use. Hand-work has 
the further advantage that in those portions of skin where the 
hair is tighter than usual it may be removed by greater pressure 
of the knife or by hand-shaving, whereas after goods have been 
unhaired by machine they must generally be examined, and any 
patches of hair removed by hand on the beam. The edges 
often require to be gone over by hand. 

Several machines with spiral knives have been introduced for 
the purpose, but the great difficulty was the rigidity of the bed 
or beam on which the hide was supported, which often led to 
damage to the grain through any lump of flesh left on the hide. 
In this respect a great improvement has been made by the use of 
pneumatic rolls, on the principle of the motor-car tyre, into which 
air is continuously forced by a pump attached to the machine, 
the surplus escaping by a relief valve loaded to the pressure to 
which it is desired to inflate the roll (see fig. 37) . 

Another successful device is the Leidgen unhairing machine, ^ 
figs. 32, 33, in which the hide is supported on an apron of stretched 
canvas, while the spiral knife-roll not only rotates, but sweeps an 
arc down the hide, which is held in position during the traverse 
of the knife by an automatic clamp. 

Occasionally goods are unhaired by fulling in the " stocks " ; 
but the saving in labour is more than counteracted by the loss 
of weight caused by submitting the hide, while its gelatin is in a 
partially dissolved condition, to such rough usage. 

The use of the wash-wheel. (see pp. 158, 165) for the same pur- 
pose is much more satisfactory, and may be profitably employed 
for common goods when the hair has been destroj^ed by painting 
with a sulphide mixture. 

1 E. H. Munkwitz, Milwaukee. 



DEPILATION 



193 



If a strong sulphide solution has been used, it may even be 
swept off with a stiff cane broom. 

After unhairing the greatest care must be taken to avoid 
exposing the hide to the air, which always contains carbonic 
acid, or still more to water containing " temporary hardness," or 




Fig. 32. — Leidgen Unhairing Machine. 



free CO2, and it is best to return them immediately to a pit of 
water which has been softened with added lime liquor, or which 
has been only partially renewed after use for a previous pack, 
and let them remain there till required for " fleshing." 

After being unhaired, the hides are " fleshed " on the beam 
or by machine. This work, which consists in removing the 
loose subcutaneous tissue and any flesh and fat left on the inner 
side of the skin, should be carefully and thoroughly done ; but 
the closeness of the fleshing required is dependent on the purpose 
to which the hides or skins are to be applied. 

13 



194 PRINCIPLES OF LEATHER MANUFACTURE' 

It is necessary not only to remove those portions of fat which 
are easily visible, but also to force out that contained in the loose 
areolar tissue. The form of knife used in England in fleshing is 




Fig. 33. — Leidgen Unhairing Machine. 



shown in fig. 34. It differs from the one used on the beam for 
unhairing in being somewhat broader and heavier, and both its 
edges are sharp, so that where the flesh is too tight to remove 
by mere friction of the knife, it may be actually cut away by 
holding the knife almost flat on the beam and using the convex 
sharp edge. The knife is ground hollow on the surface next the 
hide. The strokes in cutting must not be too broad, or from the 
convexity of the beam the substance of the hide will be cut into 
in the middle, or " flesh " will be left at the edges of the stroke. 



DEPILATION 



195 



This difficulty is avoided by the flexible knife commonly used in 
Germany, but in other ways its work is less rapid and effective. 
Machines have long been used for fleshing and scudding light 
goods, such as lamb-, kid-, and goat-skins, and their use for 
fleshing dressing hides has now become very general in the 
United States, and is gradually gaining ground in England. The 
type of machine .used for these heavier leathers varies consider- 
ably from that used -for light skins, but the general principle is 




Fig. 34. — Fleshing. 



the same. In most cases the working tool of the machine is a 
cylinder with spiral blades, which are generally arranged right- 
handed on one half and left-handed on the other, so as not only 
to scrape the hide in the direction in which the cylinder works, 
but also to extend it sideways. Much of the efficiency of these 
machines depends on the exact adjustment of the pitch of the 
spiral, and in the Vaughn machine, which is probably most in 
practical use, the blades are so arranged as to form two inter- 
secting spirals, one of steeper pitch than the other. The great 
difference in the machines for skins and for heavy work consists 
in the means adopted to support the skin and to carry it under 
the spiral blades. 

For heavy hides the Vaughn machine is most generally used, 
and may be taken as the type of the rest, as the Vaughn Company 



196 PRINCIPLES OF LEATHER MANUFACTURE 

certainly originated the semi-cylindrical " beam," which forms a 
very important feature. Its construction will be seen from 

fig- 35- 

It will be easily noticed that if a hide be thrown over the 
half-cylinder so that one half hangs outside it and the other 
half falls in its hollow, and it be then rotated, the hide is first 
caught firmly by a spring-clamp, which has been supported above 
the edge of the half-cylinder by blocks attached to the fram.e. 
As the edge rises it lifts this clamp off the blocks, and thus carries 
the hide under the spiral knife-cylinder. The blades of this spiral 
knife-cylinder are ground to a sharp rectangular edge, and partly 
scrape and partly cut the loose tissue of the flesh. When the 
half-cylinder has made a semi-revolution it returns to its original 
position, and the sizes of the driving pulleys are so arranged that 
the cylinder travels downwards more rapidly than it rises in 
order to economise time, though in both cases the hide is worked 
upon by the knife-spiral, which is rotated at a still higher speed. 
The hide is then turned on the beam-cylinder, and the other half 
is similarly fleshed. The beam-cylinder reverses automatically, 
or may be reversed by hand, and its nearness to the spiral knife 
is also under control. It is usually covered with a thick sheet of 
rubber. 

It is obvious that machines of this type can be used not only 
for fleshing but for unhairing and scudding, by the substitution 
of suitable knife-cylinders. 

The Vaughn machine is frequently, used in America for 
fleshing hides after soaking but before they go into the limes, 
and much is to be said in favour of this method, as the removal 
of the flesh permits even and uniform action of the lime. It is, 
however, a distinct disadvantage to the method that the flesh 
appears rough-looking after tanning, and the method is most 
suitable in conjunction with the American system of splitting 
the tanned leather. 

In the production of sole leather fleshing machines have not 
as yet come into very general use. This may be accounted for 
by the fact that if used before liming a rough flesh is produced, 
which is unsightly on sole leather, and which cannot well be 
afterwards improved, while something of the same objection 
attaches to fleshing after liming, with the added disadvantage 
that the hide is too much pressed, and is not easy to plump 
again so as to make a satisfactory sole leather. 

What is really wanted is a machine which will really cut the 
flesh as the knife does. A machine which did this was invented 
by the late Mr E. Wilson, in which cutters something like the 



198 PRINCIPLES OF LEATHER MANUFACTURE 




Fig. 36. — Diagram of Hide. 



blades of skew-planes were attached to an endless band, which 
worked above a cyhnder carrying the hide, and cut off the flesh 
almost Hke a splitting machine. ' It did excellent work, but was 
not much more rapid than hand fleshing. Dressing-hides are 
often fleshed in the States in the tanned condition by passing 
them through the band-knife splitter flesh side up, and with the 
machine so adjusted as to take a very thin skiving, the splitting 

proper being afterwards done in 
the ordinary way grain-side up. 
It is doubtful, but possible, that 
this might be done in the limed 
state with a machine with a 
grooved feed-roller, such as is 
used for splitting offal from the 
limes. The writer also suggested 
many years since that fleshing 
could be done by a machine like 
that used for shearing, cloth, in 
which a spiral knife works against 
a fixed ledger-blade with a scissors- 
like action. The loose flesh would 
be drawn up by the rotating knife 
and cut off, while the firmer corium 
would pass under it, but, so far as he is aware, the idea has 
not been practically tested. In any case, at the present price 
of labour, machine-work is sure to come more into use. 

In America, both sole and dressing leathers are usually tanned 
in sides, the hide being cut down the centre of the back. In 
England, the hide is usuafly " rounded " for sole leather into 
" butts " or " bends " (half butts) and " offal," as shown in 
fig. 36. The rounding is done by hand with a sharp knife on a 
table, and in some of the best tanneries frames made of wood 
or metal are employed to mark the sizes required. The chief 
advantage of rounding before tanning is that the various parts 
of the hide can be differently tanned, and appropriated to the 
purposes for which they are most suitable. The offal is now 
frequently spht either from the hmes or after tannage and worked 
up for hght leather, or in other cases is tanned with a cheaper 
and more rapid tannage than the butts. 

Dressing leather is more frequently rounded after tanning, 
according to the purposes for which it may be required. 

A considerable improvement has been made in recent times to 
fleshing, and some other leather-working machines, by substi- 
tuting for a rigid, or simply rubber-covered roller supporting the 



DEPILA riON 



199 



hide under the spiral knife a pneumatic roller on the principle 
to the motor car tyre, which is kept inflated to any required 
pressure by a pump driven by the machine, and provided with 
a safety-valve allowing surplus air to escape if the pressure is 
exceeded. A machine of this type is shown m fig. 37. 




Fig. 37. 



CHAPTER XIV 

DELI MING, BATING, PU BRING, AND DRENCHING^ 

Although lime is in many respects a most useful and satis- 
factory means of loosening hair, it is important that it should 
be completely removed, at least from the grain surface, when 
it has done its work, since its action on tannins is most injurious, 
and it is often harmful in tawing. For soft leathers it is also 
necessary that the skin should be brought from a swollen to a 
soft and flaccid condition. 

All the processes mentioned under the heading of this chapter 
have the removal of lime for one of their objects, but, except in 
that of deliming with acids, they also fulfil other functions, such 
as bringing down the alkaline swelling, and removing certain 
constituents of the skin which are not required in the finished 
leather. All the older processes did this by means of bacterial 
fermentation, but it is now understood that it was by the agency 
of the enzymes or digestive ferments which the bacteria produced, 
and several modern bating preparations utilise the tryptic or 
pancreatic enzymes derived direct from animals. 

The enzymes form a curious link between living and dead 
matter. Very small quantities are capable of breaking down 
large quantities of proteids into their simpler forms and, ulti- 
mately, into their amino-acids ; and they do this without them- 
selves apparently suffering change. They are very selective in 
their action — the tryptic enzymes attack epidermis-matter and 
emulsify fats, but are unable to attack the unaltered hide-fibre 
itself, while pepsin digests the latter, but does not attack the 
epidermis nor fats. Most natural enzymes are mixtures, and it 
is quite possible that if we could completely separate them, we 
might find that each single enzyme could only digest one species 
of proteid ; and if we knew exactly what we wished to remove 
from skins, we might select the enzymes which would do it without 
touching anything else. The enzymes are also very particular 
as to the exact degree of acidity or alkalinity of the solutions in 
which they work, and each has an optimum hydrion concentra- 

^ See also The Puering, Bating, and Drenching of Skins, by J. T. Wood. 
Spons, 1912. 



DELIMING, BATING, PUERING, DRENCHING 201 

tion ; thus the pepsins will only act in faintly acid solutions, 
while the trypsins require alkaline media. 

As regards the extreme " pulling down " effect of many of these 
fermentative processes no adequate explanation has yet been 
given, but it is a plausible, if as yet unproven, hypothesis that 
they automatically bring the alkalinity or acidity of the solution 
towards the isoelectric point (cp. p. 370) of collagen, which is the 
point of minimum swelling. ^ As the proteids are broken down 
towards amino-acids, their capacity for combining with acids or 
alkalies is vastly increased by the added number of free (terminal) 
amino-groups and carboxyls, and as the amino-acids themselves 
are very weak, either as bases or acids, they must ultimately- 
bring down the hydrion concentration of the solution to a point 
very near neutrality. No decisive opinion can, however, be given 
till much more electrometric work with the hydrogen concentra- 
tion cell has been done on these liquids. 

The older methods may be described as follows : — 

Bating consists in handling, or steeping, the goods in a weak, 
fermenting infusion of pigeon- or hen-dung for a time usually 
extending over some days, and is applied to the heavier classes 
of dressing leather, such as " common " and shaved hides, kips, 
and calf-skins. 

Puering is a very similar process, applied to the finer and 
lighter skins, such as glove- and glace-kids and moroccos, in 
which dog-dung is substituted for that of birds, and, as the 
mixture is used warm and the skins are thin, the process is 
generally complete in a few hours at most. Neither bating nor 
puering are very effective in removing lime, and seem to act 
principally by some direct effect of the bacterial products on the 
swelling of the pelt. 

Drenching is occasionally used {e.g. on calf-kid) as a substitute 
for bating or puering, but more frequently follows the latter, 
and serves to cleanse and slightly plump the skins before tanning, 
and complete the removal of lime. The drench-liquor is an 

1 Wood informs me that he has found the usual reaction of the bate is 
P„=7-o^ and the optimum concentration for the action of trypsin at 
37° C. is Ph 9-7. The isoelectric point of collagen has been shown to be 
almost identical with that of gelatin, which is about Ph=4-7 or N/50,000, 
and thus slightly on the acid side of neutrality ; but while the swelling 
curve rises rapidly on the acid side, it rises only very gradually up to 
about Ph=9 on the alkaline. Bates and puers are usually faintly acid at 
the outset, and are only rendered alkaline by the lime removed from the 
skins, and the swelling is influenced by other ions beside H+ and OH'. 
NaOH, which swells twice as much as its equivalent of Ca(OH),, is certainly 
removed in bating and puering. 



202 PRINCIPLES OF LEATHER MANUFACTURE 

infusion of bran made with hot water, and allowed to ferment 
under the influence of special bacteria, which are always present 
in vats used for the purpose, and which develop lactic and acetic 
acids. 

It will be noted that all these methods are fermentative, and 
their effect is not simply the chemical one of removing the lime, 
but the bacterial action leads also to solution of the cementing 
substance of the hide-fibres, and produces a marked softening 
effect on the leather, together with considerable loss of hide- 
substance. In the manufacture of the softer leathers this effect 
is generally desired, and no process would be satisfactory which 
did not produce it ; but in other cases, such as harness- and 
strap-butts, firmer and heavier weighing leathers would be pre- 
ferred if it were known how to make them. The putrefactive 
processes would be gladly relinquished if satisfactory substitutes 
could be found, not only on account of their offensive character, 
but because of their uncertainty and danger to the goods ; and 
even if lime only were removed, the necessary softness could 
often be obtained by appropriate liming and tanning. 

It will be best, therefore, to deal first with the purely chemical 
methods which aim only at removal of lime before considering 
those involving bacterial action. Unfortunately, the chemical 
problem is not so simple as it might at first sight appear. The 
alkaline lime clings obstinately to the hide-fibre, and can only be 
removed partially by mere washing with water. On the other 
hand, the use of any considerable excess of strong acid is pre- 
cluded, because of its powerful swelling effect on the pelt, in the 
tanning of which it would prove even more injurious than the 
lime, making dark-coloured and brittle, or tender, leather. This 
effect is not to be avoided by the use of even very dilute solutions 
of " strong " acids if used in excessive quantities, since the 
affinity of hide-fibre for them is so great that it will abstract 
practically all the acid from even a decinormal solution, leaving 
it quite neutral. What is required is an acid of extremely weak 
affinities, forming soluble lime salts, and obtainable at a low 
cost ; or, on the other hand, a salt of some weak base which 
could be displaced by lime, and which would not act injuriously 
on the pelt. With certain precautions, and in special cases, 
however, the stronger acids may be used successfully. 

In the cases of sole and belting leather no softening is desired, 
and formerly tanners usually contented themselves with a very 
perfunctory washing in water, trusting to the acids present in the 
liquors to complete the removal of the lime. Even pure distilled 
water effects this removal very slowly and imperfectly, owing to 



DELIMING, BATING, PUERING, DRENCHING 203 

the strong attraction of the lime for the fibre ; and if " temporary 
hard " water is used, the Hme present in the hide combines with 
that present in the water and is precipitated as chalk in the 
surface of the hide. This may be prevented by previously adding 
a small quantity of lime or lime-liquor to the water before use 
to soften it (see p. 68) ; but unless this is very carefully done, 
the free lime present in the water prevents it from removing any 
from the hide. The safest way is not to add lime direct to the 
water, but to change the latter gradually, so as to allow the lime 
already present to soften the new portion of water. 

Mere washing, however, is never thoroughly effective, and a 
much more efficient method is to suspend the butts in water to 
which small portions of diluted acid are successively added till 
the lime is nearly, but not quite, neutralised. If carefully used 
sulphuric acid may be employed, but, of course, any excess will 
spoil the colour or " buff " of the leather, and, unless the water is 
very soft, it always leaves calcium sulphate in the hides. Hydro- 
chloric acid, though more costly, is free from this defect, but 
must not contain any considerable quantity of iron. For lighter 
leathers the writer has proposed the use of equivalent quantities 
of sulphuric acid and common salt, which J. T. Wood {Bating, 
Puering, and Drenching, p. 11) states is perfectly satisfactory. 

For skins for puering. Wood only employs 4000 c.c. of hydro- 
chloric acid of 18° Be. (28-5° Tw.) for 250 kilo, of wet pelt, 
though this is only sufficient to dissolve about one-tenth of the 
lime present, since puering is an alkaline process. The process is 
best carried out in a paddle, beginning in cold water, and adding 
the much diluted acid very slowly, but towards the end warm 
water is run in to warm the skins before going to the puer. The 
acid should be added in at least three separate portions, say at 
intervals of ten, fifteen, and twenty minutes or longer. For 
details cp. Wood, loc. cit., p. 8. 

Acetic, formic, and lactic acids are safer than sulphuric, but 
are somewhat costly, and must not be used in appreciable excess. 
Crude pyroligneous acid may be used, and it has a considerable 
antiseptic effect owing to the phenols, etc., which it contains. 
Sulphurous acid ^ is perhaps the best, and its acid properties are 
so weak that slight excess does little harm, but . the neutral 
calcium sulphite is insoluble, and to actually dissolve the lime 
the hydric sulphite must be formed, which can only occur in 
presence of excess of the acid. Unless such excess is used, the 
colour of the pelt in the early liquors is apt to be somewhat 

1 Manufacture of sulphurous acid, see p. 24 ; testing, see L.I.L.B., p. 37, 
and L.C.P.B., p. 13. 



204 PRINCIPLES OF LEATHER MANUFACTURE 

greyish. Probably a very good method would be to suspend the 
butts in a deep pit, preferably provided with some means of 
agitation, in a solution of sulphurous or some other acid of about 
N/20 strength, sufficiently long to remove all lime from the 
surface and slightly to plump but not to penetrate to the centre 
of the hide, which should then be suspended in water until any 
excess of acid had been taken up by the unneutralised lime still 
present in the middle of the butt, which at the end of the opera- 
tion should be rather alkaline than acid. The course of this, or 
any other "deliming, operation can be followed by cutting the 
hide, and moistening the cut surface with alcoholic solution of 
phenolphthalein, which is turned red, or pink, by the least trace 
of free lime. 

In using mineral acids it is of great importance that they 
should be perfectly free from iron, and that the vat employed 
should contain no iron which could become dissolved, since, if 
present in the bating liquid, it is sure to be fixed by the hide, 
especially if the quantity of acid used is insufficient to neutralise 
the whole of the lime. 

Besides the direct use of mineral acid which has been described, 
sulphuric or, still better, oxalic acid may be very advantageously 
employed in precipitating lime from used bating liquids contain- 
ing weak organic acids or other lime solvents, so as to restore 
their original activity. Not only is the bate economised by being 
used repeatedly, but some of the organic products dissolved from 
the hide have themselves considerable power of removing lime. 
Putrefaction should not be. allowed to take place ; but many of 
the organic acids which have been proposed for bating belong 
to the aromatic series, and have considerable antiseptic power. 
Where organic acids are employed, the presence of their neutral 
lime-salts in the liquor, resulting from previous operations, will 
reduce the swelling action of the acid on the skin without 
diminishing its power of removing lime, but neutral salts of 
strong acids have little effect on the strong acids {cp. p. 100). 

Ordinary sodium bisulphate has the disadvantage of leaving 
practically the whole of the lime, in the form of calcium sulphate, 
in the hide, and therefore is only suitable for sole leather, but it 
can be used for the purpose, and its action is more mild than 
that of sulphuric acid itself, but great care must be taken that 
no nitric acid is present, as is frequently the case in the crude 
product obtained in the manufacture of nitric acid from sodium 
nitrate, and known in commerce as " nitre-cake." The presence 
of a trace of sodium chloride would not be disadvantageous for 
dressing leather, but would tend to prevent plumpness in sole. 



DELI MING, BATING, PUERING, DRENCHING 205 

Paessler and Appelius ^ have shown that raw hide absorbs sul- 
phuric acid from sodium bisulphate, leaving the neutral sulphate 
in solution. 

Boric (boracic) acid, though used to a slight extent for a 
number of years past, has recently come much into favour as a 
deliming agent, for which purpose it is in many respects par- 
ticularly suitable. Sole leather may be improved in colour by 
giving a short bath in i| to 2 per cent, boric acid solution to 
remove surface-lime. In this case the acid is best applied just 
before the hide enters the suspenders. Boric acid may also be 
suitably employed on hides which have been bated. It then 
acts as a drench and removes traces of lime still left in the hides, 
so that the liquors have a more even effect on them. Experience 
has shown that the skins should never be allowed to lie for any 
length of time in the boric acid solution in a motionless condition, 
as this tends to produce patches of partially delimed skin, which 
cause irregular colour. It is best to keep the skins in fairly 
constant motion in a paddle or by frequent handling. Boric 
acid has considerable influence in preventing drawn grain in the 
early liquors, but if it gets into the forward liquors it renders the 
leather loose and light {cp. p. 370, and L.I.L.B., p. 37, and 
L.C.P.B., p. 12). 

Borax has also been suggested as a deliming agent, and as it is 
chemically an acid salt, it has naturally some deliming effect, 
but it cannot compare with boric acid in either price or efficiency. 

Both boric acid and borax are antiseptics (see p. 26). 

In the employment of either sulphuric, boric, or any other 
acid forming calcium salts of limited solubility, it must be borne 
in mind that if the solution is repeatedly re-strengthened it will 
become saturated with the lime-salt, and although the acid will 
still combine with the lime and render it neutral, it will no longer 
remove it from the hide. Under these conditions, sulphuric acid 
may cause the deposition of crystalline calcium sulphate in minute 
nodules between the fibres. Calcium borate may be similarly 
deposited, and has the further disadvantage of becoming decom- 
posed by the tanning liquors, which form dark compounds with 
the lime. In using sulphuric or boric acid alone it is therefore 
best to renew the water each time. When it is used in con- 
junction with some other acid, forming very soluble lime-salts, 
this danger is not to be apprehended, while oxalic acid pre- 
cipitates the lime almost completely from the solution. 

It is to be borne in mind that in all cases of using acids, any 
carbonate of lime present on the pit sides or elsewhere will be 
^ Wissenschaftlich-Technische Beilage des Ledermarkt, 1901, p. 107. 



2o6 PRINCIPLES OF LEATHER MANUFACTURE 

decomposed, and the carbonic acid will become dissolved in the 
liquor, and unless acid is used in sufficient quantity to remove 
the whole of the lime, may tend to fix the remainder as carbonate. 
In the case of dressing leather there is less danger of this, as 
warm water is generally used, in which little carbonic acid dis- 
solves. It is probable that some of the coal-tar sulphonic acids 
which have been advertised for bating dressing leather might be 
advantageously employed for sole, and if the liquor were re- 
generated by the addition of sufficient sulphuric acid to neutralise 
the lime dissolved from the hide, might be used repeatedly, and 
would not then prove expensive ; while their sterilising power 
would be very advantageous to the proper swelling of the butts 
in the handlers, since nothing tends to check plumping so much 
as putrefactive action. 

Turning from sole to dressing leather, mineral acids are very 
successfully employed for " pulling down," the goods being 
thrown into a paddle containing warm water of about 30° to 35° C, 
and the calculated quantity of sulphuric or hydrochloric acid, 
previously largely diluted with water, is then added in successive 
portions at intervals, and more gradually and slowly the thicker 
the hides. The acid must in no case be sufficient to neutralise 
quite the whole of the lime. Goods treated in this way can be 
further bated, puered, or drenched as required by the ordinary 
methods if they are not sufficiently soft. If too much acid has 
been used, and the skins show signs of swelling, they must be 
brought down by the addition of a little ammonia, or even 
soda, as if swollen with acid they will not bate with an enzyme- 
bate. 

In many cases the addition of salt in small quantity to the 
acid liquor will tend to deplete the hides, and at the same time 
prevent any injurious action of the acid. Ammonium chloride 
may also be used with advantage (see p. 209). A solution con- 
taining about 15 per cent, of salt and 0-3 per cent, of sulphuric 
acid, with some molasses, has been a good deal used in the States 
as a deliming agent, and seems to answer well on some classes of 
goods, but the acid and salt are apt, ultimately, to find their way 
into the liquors and destroy tannin. The process is well suited 
for chrome leather, and may also be usefully applied in cases 
where goods have become " wind-blasted " or otherwise impreg- 
nated with carbonate of lime, since in presence of salt the acid 
can be used in sufficient excess to dissolve the carbonate. Vege- 
table acids may, of course, be used in conjunction with salt in the 
same way. The salt does not neutralise the acid, but simply 
controls the swelling of the skin, and if acid has been used in 



DELIMING, BATING, PUERING, DRENCHING 207 

any material excess, the first part of the tanning must be done 
in salted liquors, or the acid neutralised with ammonia, sodium 
carbonate, or chalk previous to tanning, as, otherwise, the 
goods will plump up in the liquors, and be tender when tanned 
{cp. P-237). 

Lactic acid has recently come largely into use as a deliming 
agent. It is best known as the acid which gives a characteristic 
taste to sour milk, and is the chief product of the lactic ferment. 
It may be very successfully used for neutralising the lime left in 
the skins after the depilation, but, if used in excess, it tends to 
plump or swell the leather. When used for deliming, a solution 
of 2 lb. in 100 gallons is very suitable. It may, in many cases, 
be substituted for the bran-drench with advantage, and is much 
more rapid and less dangerous in hot weather, but the effect is 
not in all respects identical. ^ 

When lactic acid is used for bating or drenching the operation 
should always be conducted in a paddle, and the liquid works 
more satisfactorily if it is at a temperature of 30° to 35° C. As 
regards cost, it will be found that in practice it is not appreciably 
more expensive than dung or bran. About an hour's paddling 
will generally suffice if the right quantity of acid has been used, 
but in some cases it is best to add the acid in several portions and 
take more time, and of course the thicker the hides the longer 
the time required. 

The estimation of the amount of lactic acid in the commercial 
article may be carried out by diluting exactly 9 grm. with about 
ten times its volume of water, and then titrating it with normal 
caustic soda as described in L.I.L.B., p. 16, for acetic acid, and 
L.C.P.B., p. 12. As each cubic centimetre of normal alkali is 
equivalent to -090 grm. of lactic acid, it will represent i per 
cent, of real lactic acid in the sample. If other acids are present, 
they are of course included. Commercial lactic acid is usually of 
about 50 per cent. 

It is important that the lactic acid should be free from iron ; 
a dilute solution should give no blue coloration on addition of 
either potassium ferrocyanide or ferricyanide. Acid perfectly 
free from iron is now easily obtained. 

1 On the manufacture of lactic acid by fermentation, see Claflin, Journ. 
Soc. Chem. Ind., 1897, p. 516. Campbell states that practically pure 
cultures of the lactic bacteria are obtained by continued culture in milk. 
These cultures employed as a ferment for drenches made with glucose 
have given good results in the Leeds University Experimental Tannery. 
The addition of a little used liquor will assist the fermentation, which 
should be kept slightly warm, say 30° to 40° C. 



2o8 PRINCIPLES OF LEATHER MANUFACTURE 

It has been shown in the Procter International Research 
Laboratory that the lactic anhydrides always present in com- 
mercial lactic acid are of equal value to the acid itself for deliming, 
and become converted into the acid on dilution. Their separate 
estimation is therefore useless.^ In fact Dr G. Eberle proposed 
the use of such anhydrides for deliming purposes, for which 
they would be very suitable in cases where it was desirable 
to keep down the amount of free acid to a minimum. The 
anhydrides of acetic, propionic, butyric, and lactic acid, as well as 
the lactone of y-oxybutyric acid and the lactide of lactic acid, 
are specified.- 

Formic acid in 60 per cent, solution, formed synthetically by 
the combination of carbon monoxide with caustic soda and the 
subsequent decomposition of the sodium formate so produced, 
has been brought into commerce at a cheap rate, and forms a 
satisfactory substitute for acetic acid in the deliming of hides 
and many other technical operations. Formic is about ten times 
" stronger " than acetic acid. Wood {loc. cit.) says that a mixture 
of equal parts of formic and acetic acids acts better than either 
alone. It is said that the formate of lime formed has a " pulling 
down " action, and that less than the theoretical quantity is 
therefore sufficient. 

Butyric acid, produced by the process of Dr Effront of Brussels, 
was on the market commercially before the war. It is a still 
" weaker " acid than acetic, and, as was to be expected, dissolves 
still less hide-substance, and if it can be produced sufficiently 
cheaply, should prove very valuable to the tanner. 

The following table, calculated in 191 1 on the then price of 

commercial acids, gives the cost of neutralising i lb. of lime 

(CaO) with each ; and though, of course, the present prices differ 

considerably, the relative cost probably remains much the same.. 

The very greatly heavier cost of the weaker organic acids shows 

the importance of liberating them again after use by the addition 

of sulphuric acid. The cost per lb. of lime is calculated by the 

pence per cwt. x 100 x eq. 

formula — -r- 

112 X 28 X per cent, strength. 

^ Thompson and Suzuki, J.S.L.T.C., 1918, p. 115. 

^ Verfahren zum Beizen der Haiiten, Ledertechnische Rundschau, Nr. 24, 
1910. Coll., 9, 1910, p. 372. 



DELIMING, BATING, PUERING, DRENCHING 209 



Cost of 


Acids 


TO Neutralise i 


LB. 


OF Lime. 




Acid. 


Equiv. 


K = ioo k. 


Cost 
per cwt. 


Strength. 
Per cent. 


Cost 
in 








s. 


a. 




pence 


Hydrochloric 


36-5 


Very large 


3 


3 


31-5 


1-4 


Sulphuric . 


49-0 


Very large 


4 





95-0 


0-8 


Oxalic 


63-0 


o-i 


30 


4 


99-0 


7-4 


Formic 


46-0 


0-0214 


35 





87-4 


7-0 


Lactic 


90-0 


0-0138 


26 





497 


i8-o 


Acetic 


. 60 -0 


0-0018 


18 





40-0 


10-8 


Butyric 


. 88-0 


0-00115 


21 





82-8 


8-5 


Boracic 

t : _ j^i_ _ I- 


. 62-0 


o-oooooooi 


27 





99-0 


6-5 



k is the dissociation-constant of the acid, and indicates its 
chemical " strength." Note the excessive weakness of boracic 
acid, which will not even redden litmus {cp. p. 99). 

Attention must here be drawn to a very important paper by 
Stiasny ^ on the applications of the law of mass action to deliming 
and neutralisation of chrome leather, processes similar in principle 
though apparently very different. 

Instead of acids, many neutral salts may be used to neutralise 
lime, and in sole leather it is not generally disadvantageous to 
leave the lime in the hide, so long as it is in an insoluble and 
fixed condition, and combined with an acid which cannot be 
displaced by tannin. Thus phosphates or oxalates of sodium 
or ammonium will convert the lime into insoluble phosphate or 
oxalate, setting free sodium- or ammonium-hydrate which forms 
soluble tannates and other salts which are easily washed out of 
the hide. Zinc sulphate will form sulphate of lime and zinc oxide 
in the hide, and seems worth further experiment for sole leather, 
but must be free from iron. It has some tanning effect, and has 
been used in conjunction with vegetable tannins. Alum, or 
sulphate of alumina, would similarly form calcium sulphate and 
alumina, but the tanning effect of alumina salts is too great to 
admit of their general use for bating, though they are very suit- 
able for chrome leather. Ammonium sulphate will form calcium 
sulphate with liberation of ammonia. 

For dressing leather the use of ammonium chloride is still 
more advantageous, and it is a powerful bating material, con- 
verting the lime into calcium chloride with the evolution of 
ammonia, which is a very weak base, and has but little plumping 
power, and is easily washed out. Ammonium chloride has been 



1 J.A.L.C.A.. 1912, p. 301. 



14 



210 PRINCIPLES OF LEATHER MANUFACTURE 

very successfully used in calf-kid manufacture as a preparation 
for drenching instead of puering, which was formerly in vogue. 
As, however, only about f oz. per dozen skins was employed, the 
cleansing must have mainly depended on the warm water with 
which it was used and the free ammonia evolved. Wood states ^ 
that ammonium chloride should not be used in greater concen- 
tration than 0-7 to i-o grm. per liter, or the skins become leathery 
and do not fall properly. It is always used in conjunction with 
pancreatic bates, in which it serves the double purpose of removing 
lime and of activating the trypsin ferment. 

The use of ammonium chloride as a bate was patented by 
ZoUickoffer in 1838. 

A bating liquor which was proposed by the writer, and which 
has been used with some success on harness leather, is made up 
with ^ lb. of good white ammonium chloride (sal ammoniac) 
and ^ lb. of Boakes' " metabisulphite of soda " per hide, and 
for successive packs sufficient sulphuric acid is added to 
neutralise the ammonia formed, together with a small quantity of 
metabisulphite and ammonium chloride to restore that carried 
out by the hides. It is probable that, this would also answer well 
for deliming sole leather, as it entirely removes lime without pulling 
down the hides much, and they would remain still plumper if 
ammonium sulphate were substituted for ammonium chloride, 
while the sulphuric acid might be safely increased till the liquor 
was but slightly alkaline when the bating was finished. About 
2 to 4 oz. of good white oil of vitriol is required per hide, but 
the exact quantity will depend on the mode of liming and the 
amount of washing the hides receive before going into the bate, 
and can therefore be only ascertained by experience. As no 
free sulphuric acid can exist in the liquor so long as the quantity 
of metabisulphite is maintained, there is no practical danger of 
spoiling the leather if the acid be in slight excess. The quantities 
given may in most cases be advantageously diminished, since it 
is not always advisable in practice to remove the whole of the 
lime, which in small quantity renders tannage and penetration of 
the liquor much more rapid, either by acting as a mordant to 
the tannin, or by temporarily neutralising it and diminishing its 
astringent action on the hide-fibre. 

Turning to dressing leather, we find that the use of cold water 
alone has been practically abandoned in this country, though 
the finest French calf is produced by repeated soakings in cold 
water with alternate workings over the beam, sometimes extend- 
ing to nine or more. In this case, from the lengthened exposure 
^ " Puering, Bating, and Drenching," p. 47. 



DELI MING, BATING, PU BRING, DRENCHING 211 

to waters which are only gradually renewed, it is probable that 
putrefactive action takes place, and that a sort of bating is 
effected by the decomposing products of the hide itself ; in fact, 
in many French yards bran-drenches have been introduced to 
supplement the action of the water alone. Waters differ greatly 
in their power of removing lime from skin. Slightly acid and 
peaty waters, and those in general which contain much organic 
matter, are much more powerful in reducing than those which 
are purer {cp. p. 82). 

Warm water has much more effect in removing lime than 
cold, since the heat lessens the risk of dissolved carbonic acid, 
and seems to have a direct depleting effect on the pelt. A good 
tumbling in warm soft water will remove a great deal of lime, 
and is an excellent preparation for bating, but heat must be 
used cautiously, and should never exceed 30° to 35° C. ; some 
skins, such as seals, being very readily tendered by its action, 
while others, especially sheep-skins, "will stand a comparatively 
high temperature. 

The temperature which skins will stand without injury depends 
not only on the nature of the skin, but on its condition ; skins in 
a neutral condition will stand considerably higher temperatures 
than those swollen with lime or other alkalies, or probably than 
if swollen with acid. The following table, due to E. Munro 
Payne, is an estimate of the highest temperatures which skins 
will stand without injury, but probably makes no claim to great 
exactness: — 



Limed skins 
Hide-substance 
Alum leather 
Tanned leather 
Oil leather . 
Chrome leather 
Aldehyde leather 



27"-28° C. 

40° C. 

50° C. 

71° C. 

84°-85° C. 

106° C. 

222° C. ? 



The use of a solution of carbonic acid for removing lime has 
been patented by Nesbitt,i who takes advantage of the fact that 
calcium carbonate is soluble in excess of carbonic acid (p. 67). 
The gas, which he generates as for soda water, by the action of 
acids on chalk or limestone, is received in a gasholder, and forced 
by a compressing pump into the vessel containing the hides, 
which is preferably a rotating drum lined with copper, and capable 
of bearing a pressure of about three atmospheres. The invention 
excited considerable interest on its introduction, as the gas is, 
1 Eng. Pats. 7744 and 12,681, 1886. 



212 PRINCIPLES OF LEATHER MANUFACTURE 

certainly, quite uninjurious to the hides, and it was claimed that 
it enabled the grease and dirt to be better removed than by the 
ordinary methods. Further experience has shown, however, that 
the removal of the lime is far from complete, since, for success, 
it is not only necessary to bring it into solution, but to wash 
it out with carbonic acid solution under pressure, as on exposure 
to the air solutions of lime in excess of carbonic acid rapidly 
deposit calcium carbonate. The only tannery in which to my 
'knowledge the process has been extensively used is that of Messrs 
Mossop and Garland, of Capetown, who stated that it answered 
very well for harness leather when a pure lime made by calcining 
sea-shells was used for liming, but was not satisfactory with 
ordinary stone lime. It is difficult to account for this on chemical 
grounds. Gluestuff may be treated very satisfactorily by simply 
blowing carbon dioxide, or washed and cooled . lime-kiln- or 
furnace-gases, into an open pit, in which the material is kept 
agitated. In this case, however, there is no need for the actual 
removal of the lime, so long as it is carbonated and its caustic 
character destroyed. Carbonic acid does not decompose lime- 
soap, and hence sets free no fatty acids, which, together with 
grease, are the main cause of the turbidity of glue, and the 
process therefore yields a more brilliant though darker coloured 
glue than does treatment with sulphurous acid. 

Several acids of the aromatic series have been from time to 
time recommended as deliming agents, and generally possess 
the merit of acting at the same time as powerful antiseptics. In 
this connection it may be well to mention the solution of i per 
cent, of phenol and 2 per cent, of boric acid used by Dr Parker 
and the writer for preparing and preserving skins for colour tests 
{L.I.L.B., p. 133, L.C.P.B., p. 115). This answers very well as a 
bate even when much diluted, and may be rendered cheap enough 
for use in practice by the employment of a good commercial 
carbolic acid instead of pure phenol, and the use of sulphuric acid 
to remove lime from the solution renders it capable of repeated 
employment. The carbolic acid should not be too dark in colour, 
and should be carefully dissolved, or " carbolic " stains will 
result. 

" Cresotinic acid," a mixture of impure acids obtained from 
cresols in the same way as salicylic acid is manufactured from 
pure phenol, was introduced as a bate and unhairing and deliming 
agent by J. Hauff, of Feuerbach.^ He also claimed the use of 
hydrochloric acid to liberate the acid after it had been combined 

1 Eng. Pats. 10,110 and 12,521. Journ. Soc. Chem. Ind., 1889, pp. 124, 
809 ; 1890, p. 85. 



DELIMING, BATING, PUERING, DRENCHING 213 

with lime in the dehming process. It is only soluble to the extent 
of about I in 800 of water, so that, even if used in excess, no 
dangerously strong solution is formed, but it has a tendency to 
swell slightly, and somewhat harden, the hides or skins, so that 
it is perhaps more suitable for sole than dressing leather. It has 
also powerful disinfectant properties. Hauff ^ afterwards patented 
the use of oxynaphthoic acid, the corresponding mixed naphthol- 
acids. Oxynaphthoic acid is only soluble in 20,000 to 30,000 
times its weight in water. 

A mixture of the a and j8 mono- and di-sulphonic acids of 
naphthalene has also been patented by Burns and Hull,^ and 
later Hauff^ patented a mixture of various impure siilphonic 
acids of cresols and hydrocarbons. All these patents are now 
expired. Cresol-sulphonic acids are the first stage in the manu- 
facture of Stiasny's " syntans," and are afterwards condensed 
with formaldehyde. 

All these coal-tar " bates " are to replace drenching 
rather than bating or puering, as their effect is mainly that 
of removing lime. From their antiseptic character they are very 
useful in stopping the effects of putrefactif .1 and preventing 
ferments being carried into the tanning liquors, and skins may 
safely be kept at least for some days in weak solutions, but any 
necessary fermentative puering or bating should usually be done 
before and not after their use. If used before puering they would 
no doubt stop any direct bacterial action on the skin, if not in 
the puer liquor, but they probably would not prevent the action 
of the bacterial enzymes already formed, and their use might 
afford a means of using bacterial enzymes without the danger 
inseparable from actual bacterial puering or bating. 

A writer in the " Gerber," 1875, p. 279, recommends the use of 
dilute solution of sulphide of sodium as a bating agent. Possibly 
it removes lime as sulphydrate, and the writer named seems to 
have obtained good results with glove lamb-skins, but experi- 
ments made at Leeds were not successful. Possibly what he 
really used was a polysulphide. Polysulphides, such as " liver of 
sulphur," or the yellow solution obtained by boiling dilute sodium 
sulphide or sodium hydrate solution with excess of sulphur, have 
great power of " bringing down " the pelt, and seem well worthy 
of experiment as bating agents. 

^ Eng. Pat. 14,889, 1888. Cp. also Journ. Soc. Chem. Ind., 1889, p. 954. 

^ Burns and Hull, Eng. Pat. 8096, 1891 ; Journ. Soc. Chem. Ind., 1892, 
p. 48. 

=* J. Hauff, Eng. Pat. 22,546, 1894 ; Journ. Soc. Chem. Ind., 1895, P- 170 ; 
Gerber,^i895, p. 133. 



214 PRINCIPLES OF LEATHER MANUFACTURE 

In India the pods of the babool {Acacia arahica) are much 
used as a bate, the infusion being allowed to ferment. In their 
dry state they contain about 12 per cent, of an easily changeable 
tannin, which does not precipitate lime-water, and which by 
fermentation is very probably converted into gallic acid. The 
use of gallic acid itself as a bate has been patented by Albert 
Hull,^ and would undoubtedly accomplish the removal of the 
lime if used in sufficient quantity ; but as he only uses a solution 
of 25 mgr. per litre (one part in 40,000), any effect must be mainly 
due to the washing with water. Gallic acid forms dark oxidation 
products with lime. 

Of the fermentative methods of removing lime, " drenching " 
with fermenting bran-infusions is the simplest in theory, and has 



t . 



•r * 









J' 



Fig. 38. — Bacterium furfuris a. Fig. 39. — Bacterium furfuris (i. 

been very carefully investigated by Mr J. T. Wood.^ It will, 
therefore, be convenient to consider this process first, although 
it is frequently employed as a means of cleansing and slightly 
plumping the skin after the lime has been removed by puer- 
ing or bating. In calf-kid manufacture, however, it is used 
without previous puering, and in some other cases it is substituted 
for the use of dung bates. The most important of the active 
ferments are two species of bacteria, named by Wood Bacterium 
furfuris a and [3, which are very similar in their form and action 
(see L.I.L.B., p. 264), but produce a somewhat better fermenta- 
tion together than separately. They are shown in figs. 38 and 39. 
Neither species has any direct action on the hide-substance, but 
ferments the glucose produced by the action of the cerealin of the 
bran on the starch which is present. A considerable quantity 

1 Eng. Pat. 14,595. 1889. 

2 Journ. Soc. Chem. Ind., 1890, p. 27; 1893, p. 422; 1897, p. 510; 
Brit. Assoc. Rep., 1893, p. 723. See also The Puering, Bating, and Drenching 
of Skins, by J. T. Wood. E. & F. N. Spon, Ltd., 1912. 



DELIMING, BATING, PUERING, DRENCHING 215 

of hydrogen, with carbon dioxide, nitrogen, and small quantities 
of hydrogen sulphide, are produced during the fermentation, 
together with lactic and acetic and traces of formic and butyric 
acids and amines. Active drenches contain i to 3 grm. of mixed 
acids per litre, to which they owe their action, a perfectly satis- 
factory drenching being produced by an artificial drench con- 
taining 0-5 grm. of glacial acetic acid and i grm. of lactic acid 
(sp. gr. I -210) per litre, in which the skins were worked for one 
and a half to two hours, while twelve to sixteen hours would have 
been required in the ordinary drench. An experimental drench 
gave the following results on analysis : — 

Formic acid .... 0-0306 grm. per htre. 

Acetic acid ..... 0-2402 ,, 

Butyric acid .... 0-0134 ,, 

Lactic acid 0-7907 „ 



Total . . . . 1-0749 y> 

It is probable that other organisms are capable of producing 
similar fermentations, and it is not certain that in all tanneries 
the same ferments are present. Mr A. N. Palmer stated that at 
the Cambrian Leather Works at Wrexham he was unable to 
detect lactic acid in the drenches, all the acids present being of 
the acetic series. 

The drench-ferments investigated by Wood are incapable of 
attacking or injuring the hide, and, in his opinion, when the skin 
is attacked, it is generally due to putrefactive and gelatin- 
liquefying organisms introduced from the bates, or from the air 
in hot sultry weather. Drenching takes place most safely and 
satisfactorily at temperatures not exceeding 30° to 35° C, when 
the process is usuaUy complete in twelve to twenty-four hours. 
In sultry weather a butyric (?) fermentation of an active character 
sometimes suddenly takes the place of the normal one (Ger. 
Umschlagen) , the skins swell rapidly, become translucent [gldsig), 
and finally dissolve to a jelly. If tanned in the swollen condition, 
tender and useless leather results, and the injury, once begun, 
proceeds with alarming rapidity, skins being sometimes completely 
ruined in a few hours. Prompt action is therefore necessary, and 
the first step to take is to add salt, which checks the fermentation, 
and acts in the same way as in the pickling process, controlling 
the action of the acid, and producing a sort of tawing. Such 
skins will yield sound leather, though the grain is apt to be 
somewhat drawn. If the skins can be immediately got out of 
the drench, the acid may be neutralised by the cautious addition 



2i6 PRINCIPLES OF LEATHER MANUFACTURE 

of ammonia, soda, or whitening to the water in which they are 
placed, preferably in a paddle, and if they are insufficiently 
drenched they may then be paddled in tepid water, though this 
is hardly likely to be needed, as the effect of the acid is to remove 
the lime very completely. The objection to the use of whitening, 
which otherwise is the safest and best material to employ for 
removing acid from pelt, is that it is apt to become mechanically 
fixed in the grain, and thus to produce bad colour with vegetable 
tans. For white or chrome leather it would do no harm. Pre- 
cautions to prevent the recurrence of the injury are to keep the 
temperature of the drench low, and to free the bran from flour 
by washing in two or three cold waters before adding to it the 
hot water with which the actual drench-liquor is made, since the 
flour, or at least its starch, is the source from which the butyric 
acid, as well as the lactic, is formed. This trouble is rare in the 
comparatively cold climate of England, and indeed the writer 
has never seen a case, or had the opportunity of investigating one. 
It is hardly probable that it is really due to butyric acid, since it 
is now known that butyric acid is an extremely mild one, milder 
than acetic or lactic, though of course in very large quantity it 
could produce the effect. It may be due to the production of 
some other acid, or possibly of some bacterial peptic enzyme, 
which liquefies and digests the hide-fibre. In cold weather, 
where drenching is proceeding in a normal way, the flour is useful, 
since it is the natural nutriment of the drench-ferment ; and, in 
England, flour is frequently added purposely to the bran to 
increase the activity of the drench. To retain the flour, the bran 
may be washed first with boiling water, which gelatinises the 
starch and makes it adhere to the bran, and, according to Eitner, 
removes a sticky fat -like matter from it, and fits it better to 
remove the fat of the skin. After soaking in hot water for two 
hours it is washed in several cold waters and infused at about 
40° C. for use.i Many tanners use the bran without previous 
washing, but if much flour is present it rises to the top with the 
gas evolved by the fermentation, and forms a pasty mass on the 
skins, which interferes with even drenching. 

The quantity of bran used in ordinary drenching is very 
variable, but about 4 parts per 1000 of water used and from 5 to 
10 per cent, on the weight of pelt may be taken as an average 
quantity, more being frequently employed. The temperature 
may vary from 10° up to about 30° to 35° C, and the time in- 
versely from days or weeks down to two or three hours, accord- 
ing to the temperature of the drench, the amount of ferment 
1 Gerber, 1882, p. 246. 



DELIMING, BATING, PUERING, DRENCHING 217 

present, and the thickness and character of the skins. The skins 
are usually thrown into the freshly prepared drench, to which 
a few pailfuls of old drench-liquor is frequently added as a 
ferment. Fermentation soon sets in, and the gas evolved causes 
the skins to float to the surface ; this is called the " working " of 
the drench. Thin skins may be sufficiently drenched after once 
rising, while thick ones require to be put down two or three 
times. A certain sign of sufficient drenching is the appearance 
of small blisters on the grain, caused by the evolution of gas in 
the substance of the skin. When these are seen the drenching 
should be at once discontinued, as otherwise the blisters will in- 
crease in number and burst through the grain, causing minute 
holes or " pricks " (one of the many forms of the complaint called 
in German Pikiren or Piquieren). When a bubble of air is enclosed 
in a fold of the sufficiently drenched skin and pressed, it raises 
the grain without actually separating it from the substance of the 
skin. The properly drenched skin also falls easily in folds when 
held between the hands either lengthways or crossways, and if 
thin the skin tightly stretched over the hand shows grains of 
bran underneath it as Uttle lumps, round which the skin clings 
to the hand. The drenched skin should not be transparent, but 
white and soft, and when pressed should retain the mark of 
the finger. Some experience is required to determine certainly 
the point of sufficient drenching, which, of course, varies with the 
character of the skins and the kind of leather which is to be 
produced ; and the feel of the skin to a practised hand is one of 
the most important criteria. 

A writer in the " Gerber " ^ divides drenching for glove-lamb into 
three classes . " sweet," " alcoholic," and " sour." Sweet drench- 
ing is done in a bath of tepid bran-water, made by infusing in hot 
water and drawing the clear liquor off the bran, which settles to 
the bottom. The skins are only allowed to remain in two or 
three hours, or not long enough for fermentation to set in. The 
process is only suited for very thin or soft skins which will not 
stand any further loosening. The use of bran-water has the 
advantage of saving the labour of " branning," or removing 
adhering bran with the knife on the beam, but it is doubtful if 
unfermented bran has much actual effect. Bran-water can, how- 
ever, be used for drenching by fermentation, and for small glove- 
lamb has largely superseded the older method. The mechanical 
action of the bran in cleansing the pelt is however often useful. 
In sour drenching the bran is allowed to steep and soften in cold 
water for many hours, and boiling water is then added till the 
^ Gerber, 1888, p. 257, 



2i8 PRINCIPLES OF LEATHER MANUFACTURE 

temperature is raised to 75° C, and it is allowed to infuse with 
frequent stirring for some hours, and after cooling to 45° a con- 
siderable quantity of old drench-liquor is added as a ferment. 
If the drench is used warm (30° to 35°, or in cold weather, even 
40° C.) the skins only remain in one to three hours, but if cold the 
drenching can be extended over a period of two to three days, the 
skins being frequently handled. This modification is suitable for 
glace-kid and the harder sorts of skins, but glove-lamb are always 
treated by the warm and rapid process. What the writer in the 
" Gerber " describes as the " alcoholic " bran-drench is probably 
the method of fermentation investigated by Wood, in which 
inflammable gases, but no alcohol, are produced. 

A normal drench plumps the goods slightly, but if it contains 
much of the putrid ferments carried in from the bate or puer 
the skins fall in it as they would do in a bate. To increase this 
effect, putrid soak4iquor is sometimes added to the drench, but 
with doubtful advantage. 

In drench-liquors the total acidity may be determined by 
titration with lime-water or N/io caustic soda, with phenol- 
phthalein as indicator ; and the volatile acids may be distilled 
off as described under the analysis of tanning liquors {L.I.L.B., 
p. 126). For more complete metho'ds of analysis the reader is 
referred to Wood and Willcox's paper on the " Nature of Bran 
Fermentation." ^ 

Drenches are said to " work " somewhat better if made with 
water containing nitrates, and this is quite probable ; but the 
necessary nitrogen can easily be supplied if required by the 
addition of a very small quantity of saltpetre. 

Wood is of the opinion that the ferments found in bran do not 
originate in the drench itself, but come from the bated skins, as 
the drench-bacteria soon die out without finishing the fermenta^ 
tion, and constant renewing of the nutrient material is necessary 
{cp. p. 18). 

Bating and puering, though differing practically in many ways, 
are identical in theory, and most of what follows applies to both 
of them. The action is much more complex than that of the 
drench, involving both chemical reactions and those of organised 
and unorganised ferments, and it is a matter of no little difficulty 
to say what proportion of the observed effect should be ascribed 
to each of these agencies. 

Formerly, the principal effect was attributed to organic salts 
of ammonia and its homologues, and to amino-acids which com- 

1 Journ. Soc. Chem. Ind., 1893, p. 422. Cp. also Puering, Bating, and. 
Drenching of Skins, p. 233. 



DELIMING, BATING, PUERING, DRENCHING 219 

bine with lime. Phosphoric acid is also present, and if any 
exists in the form of soluble salts, it will combine with lime, and 
render it insoluble and inactive. It is probable, however, that 
most if not all the phosphoric acid is already in the form of 
tricalcium phosphate, and therefore without effect. 

It is now, however, recognised that the effects of these chemicals 
are of no importance as compared with the products of bacterial 
action, and the researches of Wood have cleared up much that 
was until recently quite inexplicable. ^ 

Much effect was formerly ascribed to the digestive ferments, 
such as pepsin and trypsin, which are present in fresh dung. It 
is known that the animal organism secretes these in considerable 
excess of its requirements, but it is doubtful whether any exist 
undecomposed, even in fresh dung ; though they are apparently 
more resistant to putrefaction and decomposition than would a 
priori have been expected of such complex organic compounds, 
and there is therefore a possibility of their existence in the dung, 
even as it comes to be used in the tannery. Both pepsin and 
trypsin are enzymes (see p. 15), and are possibly proteins. 
They are soluble in water but insoluble in alcohol, and 
hence are precipitated by the addition of the latter to their 
solution, but are not altered by it, and regain their activity on 
solution in water. Wood separated puer enzymes in this way 
fifteen years ago. They have been kept in a dry state, and are as 
active to-day as when first prepared. By heat they are coagulated 
and decomposed, and their activity permanently destroyed. 

Pepsin is the active principle of the secretion of the glands of 
the stomach, and large quantities are prepared for medical use 
as an aid to digestion from the stomachs of pigs. Pepsin only 
acts in slightly acid solution, and though fresh bate liquor is 
slightly acid to litmus, it speedily becomes alkaline from the 
lime of the skins and the ammonia present, so that the action of 
pepsin in a bate can only be a very limited one. Wood ^ com- 
pared the action of a i per cent, solution of pepsin, acidified with 
0-2 per cent, of hydrochloric acid, with that of a dogs'-dung puer 
liquor, both at the temperature of 40° C. At the end of one hour 
the skin in the pepsin-solution was considerably fallen, but that 
in the puer solution was almost dissolved. Since the solution 
here employed was much stronger than is likely to occur in 
practice, and the conditions much more favourable to its action, 
it may be assumed that the practical effect of traces of pepsin 

^ Journ. Soc. Chem. Ind., 1894, p. 218 ; 1895, p. 449; 1898, pp. 856, 
loio ; 1899, PP- 117. 990; 1912, p. 1105. 

^ Ibid., 1894, p. 220. Cp. also Puering, Bating, and Drenching. 



220 PRINCIPLES OF LEATHER MANUFACTURE 

from the stomach in the bate may be neglected. The peptic 
enzymes of the larger intestine, which are more likely to be found 
in dung, are bacterial in their origin, and though o-2 per cent, of 
HCl is the optimum for the pepsin of the stomach, it is too high 
for those of the intestine. 

Trypsin or pancreatin,^ if present, is more likely to have an 
effect, since it is active in neutral and in alkaline solutions. It 
is the product of the pancreas, and is largely concerned in in- 
testinal digestion. Chemically it much resembles pepsin, but is 
more resistant to heat, retaining its power of digestion after 
heating to a temperature of i6o° C. in a dry condition. Its 
warmed solution dissolves fibrin almost instantly and in 
large quantity, and peptonises gelatin so as to render it 
soluble in water. Wood found that a i per cent, solution 
of pancreatin acted far more rapidly than a solution of pepsin 
of equal strength. At 40° C. in neutral solution the skin fell 
rapidly, and the action continued even in the cold. In fifteen 
hours the liquid was swarming with minute bacteria. At the 
suggestion of the Author the experiment was therefore repeated, 
with the addition of 15 per cent, of chloroform, which prevented 
the development of bacteria while it did not stop the action of 
the pancreatin. The skin fell as before, but in neither case had 
it the peculiar touch of puered skin, nor were the characteristics 
of the leather produced from it the same. - Wood found later that 
the unsatisfactory result of 'this experiment was due to the 
presence of excess of lime and the want of ammonia, which is 
necessary to activate the pure pancreatic ferment. In the body 
this is done by a special enzyme, enterokinase, which is secreted 
in the small intestine. The practical pancreatic bates, such as 
" pancreol " and " oropon," contain ammonium chloride. This 
addition was due to Dr Rohm, who patented oropon in this 
country, but the principle was covered in America by a previous 
patent of Wood's. 

It is certain, however, that fresh bird-dung, and probably that 
of all animals, contains ferments capable of liquefying gelatin. 
An instance of this is found in the observation, common in glue 
manufacture, that if the dropping of a sparrow falls on a cooler 
full of solidified gelatin size, it will liquefy a track quite down to 
the bottom of the cooler. Trypsin, or at least the secretion of 
the pancreas, as well as the gall from the liver, have great power 
of wetting and emulsifying fats, and this has possibly something 
to do with the action of the bate in enabling the skins to be 
cleansed of fat. 

^ Loc. cit. and Beilstein, iii. p. 1308, 2nd edition. 



DELIMING, BATING, PUERING, DRENCHING 221 

Bacterial fermentation and its products are, however, the main 
factor in the action of puers and bates, and on this subject we 
owe most of our knowledge to the work of J. T. Wood, since, 
though Popp and Becker have worked over much of the same 
ground, they have not nearly so freely published their results. 

Wood showed that a fresh puer liquor, even when boiled for 
half an hour, and so freed from living organisms and albuminoid 
ferments, has still considerable action on a limed skin, though 
much less than the unboiled puer. He found that this action was 
principally due to amines and their compounds with organic 
acids, which removed lime, but did not remove the interfibrillary 
substance or give the proper feel of puered skin. A very similar 
result was obtained with aniline hydrochloride in i per cent, 
solution. 

A considerable variety of bacteria from dung and other sources 
were cultivated in various media and their puering power tested, 
but though greater than that of the unorganised chemical com- 
pounds such as amine salts and organic acids, it was in no case 
equal to that of an ordinary puer, or sufficient for practical use. 
When, however, a small quantity of the amine salts obtained 
from the puer was added to a mixed bacterial culture the effect 
on the skin was almost as rapid and considerable as with an 
actual puer. 

In order to determine whether the puering effect was due to 
the direct action of the bacteria or to their enzyme-products, the 
latter were separated from a filtered puer solution by adding it 
to a large volume of 98 per cent, alcohol, in which the enzymes 
are insoluble. When redissolved in water they had a decided 
puering effect, and a solution of 0-5 grm. of the mixed enzymes 
and 0-5 grm. of the mixed amine hydrochlorides in 100 c.c. of 
water at 35° C. brought down a piece of limed sheep-skin in 
thirty minutes exactly like a puer. The action is therefore 
dependent on the mutual action of the enzymes and amine 
salts, but as the separation of these would be too costly for 
practical use, and the puering seemed effectual when they were 
formed in contact with the skin by active bacteria, Wood 
adopted the method of preparing a suitable sterilised nutritive 
liquid, which was inoculated before use with a mixed culture of 
suitable bacteria. For laboratory purposes a suitable culture- 
medium was obtained by digesting 10 grm. of gelatin with 5 grm. 
of lactic acid (reckoned water-free) and 100 c.c. of water for 
three hours in a closed vessel on the water-bath. The resultant 
solution was neutralised with sodium carbonate and diluted to 
I litre with addition of a small quantity of potassium phosphate. 



222 PRINCIPLES OF LEATHER MANUFACTURE 

The bacteria of fresh dog-dung were not found to produce a 
satisfactory puering effect, but those from dung which had been 
fermented a month (as in practice) gave a result nearly equal to 
actual puer. A still better result was obtained by a mixed culture 
from the roots of wool loosened by sweating. The bacteria were 
principally of two species, of which neither separately was capable 
of satisfactory puering, but which together acted more rapidly 
than an actual puer. These bacteria do not liquefy gelatin. 

During the course of his experiments Wood found that filtered 
puer solutions were less active than turbid ones, and that their 
activity was increased even by the addition of inert substances, 
such as kaolin. 

.Wood attributes the differences in action between dog-dung 
and bird-dung not only to different bacteria, but to the fact 
that in the latter case the urinary products, and especially uric 
acid, are contained in the dung. 

From the results of these and similar researches. Wood in 
England and Popp and Becker in Germany succeeded in pro- 
ducing a practical artificial puer, which they manufactured in 
conjunction under the name of " Erodin," consisting of a solid 
nutrient medium and a liquid " pure culture " of the bacteria 
necessary to effect the required bating or puering. 

The preparation was quite successful, and superseded dog-dung 
in many tanneries, though for sheep- and goat-skins it never quite 
took its place, but for calf it proved much safer and less liable 
to cause stains. Details of its use are omitted, as it has been 
almost entirely replaced in its turn by pancreatic preparations, 
which are still safer, and can be better modified to suit the 
leathers which are being produced. 

The pancreatic bates were suggested by Wood's work on the 
pancreatic ferment, which has already been mentioned, but though 
he included its use in an American patent, he did not at the 
time pursue the idea, thinking that the use of pure cultures of 
bacteria offered better prospects of success. His idea was, how- 
ever, taken up by Dr Rohm ^ in Germany, who produced the 
very satisfactory bate " ofopon," and a similar preparation, with 
some improvement in detail, is now being produced under the 
name of " Pancreol." 

The instructions for using " Pancreol " for calf- and sheep-skins 
intended for either vegetable or chrome tannage are as follows : — 
The limed skins are partially delimed either by thorough washing 
with soft water or by the use of weak organic acids. The skins 
are then warmed up and partly bated in a weak " Pancreol " 
^ Ger. Pat. 200,519, 1908. 



DELI MING, BATING, PUERING, DRENCHING 223 

bath made by diluting half the liquor used for the last pack, with 
hot water. The temperature should be 95° F., and the skins are 
paddled for fifteen minutes. The main bating liquor is prepared 
by diluting the remaining half of the old liquor with hot water so 
as to bring the temperature to 98° F. The required amount of 
" Pancreol " (about 9 to 12 oz. per 100 lb. of pelt) is then added 
and stirred in for five minutes. The goods are then introduced 
and paddled for twenty minutes. Afterwards the skins are 
paddled for a few minutes every quarter of an hour until they are 
sufficiently bated. This occupies one to four hours according to 
requirements. The amount of liquor used should not be more 
than 25 to 30 gallons per 100 lb. of pelt. After bating, the skins 
are carefully scudded, and washed in warm, clean water. Subse- 
quent treatment is just the same as with skins bated with excre- 
ment bates. Users are warned not to dissolve " Pancreol " in 
boiling water. This is of course very injurious to the enzymes. 
The above instructions require modification for other classes of 
skins, etc., but there is no change in principle. 

In addition to trypsin, the pancreas also secretes the enzyme 
steapsin, which has the effect of emulsifying and saponifying fats, 
but like trypsin is inert as secreted, and requires to be energised 
by ammonia or enterokinase. As these enzymes only act in 
alkaline solution, care must be taken, if the skins are delimed 
with acids before the bate, not to carry the action too far, but to 
leave some lime in the skin. 

These pancreatic bates are being more and more widely used, 
sometimes with the addition of fat-emulsifying enzymes, and with 
varying quantities of ammonium chloride to remove lime, and 
are likely as their use is more fully understood to take the place 
of fermentation-bates and puers for all kinds of leather. 

Considerable light has been thrown on the real object of puering, 
which obviously accomplishes more than the mere removal of 
lime and of the remains of the hair follicles and fat -glands, by the 
work of Rosenthal and Wilson in America,^ Moeller in Germany,^ 
and Seymour- Jones ^ in this country, who have pointed out that 
probably its most important' object is the digestion and removal 
of the elastin fibres, which are very abundant in the grain layer 
(p. 63) and prevent its stretching. Figs. 40-43 are from photo- 
graphs by Mr Guido Daub of Messrs Galluns' laboratory of 
Milwaukee, and are sent by Mr J. Wilson, their chief chemist. 

1 J.A.L.C.A., 1916, p. 463; /. Ind. Eng. Chem.. 1920, p. 1087; 
J.S.L.T.C. 1921, p. 15. 

^ Collegium, 1918, pp. 108 and 125. 

2 J.S.I..T.C., 1920, p. 60. 



224 PRINCIPLES OF LEATHER MANUFACTURE 

Seymour- Jones has also shown that in some cases it is sufficient 
merely to paint the grain with the enzyme-bate, without acting 
on the fibrous part of the corium, which contains little or no 
elastin. 

In addition to the artificial bates which depend on the pan- 
creatic enzyme, a considerable number of preparations of a secret 




Fig. 



40. 



-Butt of calfskin bated with ammonium chloride only, 
stained with Weigert's magenta. 



Elastin 



nature are on the market, most of which are drenches or 
deliming agents rather than true bates, though for certain classes 
of leather they may serve a useful purpose. Many of them 
contain glucose or sugars, and some lactic or butyric ferment. 
Perhaps an exception should be made to a patent by the late Dr 
Eberle of Stuttgart,' who employs gall and extracts of the 
intestines in addition to the pancreatic ferments. These contain 
enzymes which activate the trypsin and steapsin, and so render 
^ Eng. Pat. 21,202, 1909. 



DELI MING, BATING, PU BRING, DRBNCHING 225 

the addition of ammonium salts unnecessary. A good deal of 
detail about this and the other advertised bates will be found in 
Wood's Puering, Bating, etc., chapter viii. 

As the old-fashioned puering method with dog-dung is still in 
considerable use, some further details of its action are necessary. 

From the multiplicity of germs present, and the adaptability 




Fig. 41. — Butt of calf-skin after six hours' bating with trypsin. Stained 

as before. 

of the dung infusion as a nutrient medium for any putrefactive 
organisms which may gain access to it, the bating and puering 
process is necessarily a dangerous one for the goods, always 
leading to loss of weight, and, if the process is carried on too 
long, to the more or less complete destruction of the skins. Loss 
of weight, however, in greater or lesser degree, is inevitable, and 
indeed necessary where a soft leather is to be produced. If the 
skins are allowed to lie in the bate or puer liquor, mud, containing 
organisms, and zoogloea-forms of bacteria settle in the folds, and 

15 



226 PRINCIPLES OF LEATHER MANUFACTURE 



produce marbled markings, streaks, and lines by the destruction 
of the grain surface (hyaline layer). Black or bluish stains are 
also often produced, known as bate-stains, and are either due to 
bacterial pigments or, in some cases, to the action of evolved 
hydrogen sulphide on iron present from salting or other sources. 
Frequent change of position is therefore necessary, especially 




Fig. 42. — Butt ofcalf-skin after twenty hours' bating'with trypsin. Stained 
' as before. 

when the liquor is active from being used at a high temperature, 
but it does not seem to be desirable to keep the skins in con- 
stant motion, as "weak grain" may be produced by the 
mechanical friction, and if puering is done in a paddle, it 
should only be run at intervals. 

T. Palmer ^ determined in experiments on pigeon-dung bates 
that there is considerable loss of nitrogen during the process, 
and recommended bating in pits from which the air was excluded 

^ Leather Trade Circular, 22nd Sept. 1891 ; 1887, p. 667 ; and Sanford, 
lourn. Soc. Chem. Ind., 1893, p. 530. 



DELIMING, BATING, PUERING, DRENCHING 227 

as much as possible, both as effecting a considerable economy in 
the dung and in excluding false ferments, which, he concludes, 
are mostly aerobic. It is not improbable that the method is 
advantageous, since it has been shown by Roscoe and Scudder 
that liquefaction of gelatin only takes place in presence of oxygen. 




Fig. 



43. — Butt of calf-skin after twenty-four hours' bating with trypsin. 
Stained as before. 



and its partial exclusion would therefore lessen the risk of over- 
bating, and consequent damage and loss of weight. 

Starting from the presumption that bating and puering are, 
in the main, bacterial processes, more or less successful attempts 
had been made previous to those of Wood, Popp and Becker, to 
substitute other fermenting substances for dung ; and probably 
these efforts failed in many cases, not so much because they 
were wrong in principle as from want of knowledge of the 
necessary details, such as the use of proper ferments, and the 
provision of suitable culture-media. Guano, prepared horse-flesh, 
urine, yeast, and fermenting vegetables have all been tried. A 



228 PRINCIPLES OF LEATHER MANUFACTURE 

solution of glucose or treacle of about lo per cent., to which 3 per 
cent, of pasty dog-puer is added about a week before use, was 
tried many years since in a morocco factory, at the suggestion 
of the writer, as at least a partial substitute for puer, and remained 
long in use there. The mixture kept for some time in an active 
state, and was added to the puer liquors in the same way and in 
approximately the same proportions as the dung paste. Similar 
in principle was the solid bate supplied by an American firm, in 
which glucose was mixed with a small amount of nitrogenous 
matter and phosphates, together with a lactic ferment, and 
which only requires dissolving in warm water some little time 
before use. Its results were good for some purposes, but rather 
resembled those of a drench than a bate. In a similar way, puer 
may be added to bran-drench liquors, and induces in them a 
fermentation which brings the skins down much lower than the 
ordinary drench. It is probable that a weak glucose solution, with 
traces of mineral constituents similar to Cohn's solution (see 
L.I.L.B., p. 269) and " set " with sour milk or fermenting drench- 
liquor, might in some cases be used with advantage for drenching, 
with a saving of cost. A writer in Hide and Leather describes 
a bate in which 2 parts by weight of glucose are dissolved in 
about 25 parts of water, and fermented, for about three days, 
with about I part of old bran drench -liquor, or o-i part of 
pressed yeast, till a foam gathers on the top, and then made up 
with water to 1000 parts ; the goods bated twenty-four to thirty- 
six hours at a temperature of about 35° C, and the bate 
strengthened for a second pack with about one-fifth of the original 
glucose, a new bate being made at the end of a week and set with 
one part per thousand of the old one. A short bating of, say, ten 
hours produced very nice harness leather, but the general tendency 
was to make the goods looser and more spongy than a dung-bate. 
It is obviously not a matter of indifference whether old drench 
or yeast is used to start the fermentation, since in the latter 
case only alcohol could be produced directly by the ferment 
introduced, though this might be fermented later by other acci- 
dental organisms into acetic acid. These mixed bates containing 
glucose are, however, probably wrong in principle, since the true 
puering and bating bacteria will not thrive in presence of acids 
and require nitrogenous nutriment, and if such bates are service- 
able at all, they act as drenches rather than as true bates. 

As regards the relative effect of dog- and hen- or pigeon-dung 
bates, the chief of the published experiments are those made by 
W. J. Salomon at the Vienna Versuchsanstalt fiir Lederindustrie,i 
1 Tech. Quart., 1892, v. p. 81. 



DELIMING, BATING, PUERING, DRENCHING 229 



in which he determined the relative solvent power of equal 
quantities as being, for dog-dung 2|, for pigeon-dung 2, and for 
hen-dung i. It is obvious that these figures, though interesting, 
must be taken with some reserve, as the composition even of 
pure dungs is by no means constant, depending on the feeding 
of the animals, and adulteration is common. The writer has 
heard stories of a certain dealer who used to fabricate his product 
from clay by the aid of a popgun, though he does not vouch for 
the statement ! It is generally held that the action of bird- 
dung is more penetrating but less softening and loosening than 
that of dog-dung, which is thus generally used for descriptions 
of leather where great softness and stretch are required. It is to 
be remembered in this connection that bird-dung bates are 
generally used cold, and hence are much slower in their action, 
which allows them time to penetrate thicker hides more uniformly. 
Few analyses of the dungs used in leather manufacture have been 
published, and these mostly with a view to manurial value. 
Schulze 1 gives the result of forty analyses of pigeon-dung as 
foUows : — 

Min. 
Per cent. 

Water .... 380 

Nitrogen . . . .1-47 

Phosphoric acid . . . i-oo 

Potash .... 071 

One sample contained 43-3 per cent, of sand 
Wood^ quotes the following: — 



Max. 


Mean. 


Per cent. 


Per cent 


40-00 


2100 


5-04 


2-53 


277 


179 


2-57 


1-46 



Hen 


-dung. 


Per cent 


Water 


. 6o-88 


Organic matter ^ .' . . . 


. 1922 


Phosphates ..... 


• 4-47 


Calcium carbonate and sulphate 


• 7-85 


Alkaline salts ..... 


109 


Silica and sand ..... 


. 6-69 


Dog-dung. 




Water ...... 


. 31-0 


Ca . . . . 




. 43-0 


Na, K, Mg . 




. 0-8 


PO4 . . . . 




• 3-4 


CO2 . . . . 




• 7-5 


Organic matter . 




. 14-2 


Traces Fe, CI, Si, loss 


•> r. c__ /"r.„ 


o-i 



Der Landwirt, i8g^,li. p. :^oi. ^ Journ. Soc. Chem. Ind. ,iSg/^, -p. 220. 

Containing nitrogen equal to 0-74 per cent, of ammonia. 



230 PRINCIPLES OF LEATHER MANUFACTURE 

This was apparently a sample from a dog fed on bones ; that 

from the kennels, which is more commonly used in leather manu- 

■ facture, contains much less lime ; a sample analysed by Wood 

gave 4-7 per cent, mineral matter, g-y'pev cent, organic, and 85-6 

per cent, of water, part of which was no doubt added. 

The quantity of hen- or pigeon-dung used in bating hides is 
very variable, but may be stated at from 12 to 60 litres per 
1000 kilos, of raw hide in at least 2000 litres of water. The bate 
is generally used cold, the hides remaining in it four to eight days, 
with frequent handling ; but some tanners, especially in the 
United States, prefer bating in a paddle or drum at a temperature 
of about 35° C, in which case the time must be diminished to a 
few hours. The dung is best infused with warm water in a 
separate vessel ^ and allowed to ferment for at least a week without 
use, when it will be found to swarm with micrococcus-chains. 
Only the clear liquor should be run into the bate-pit, the sediment 
and dirt being thrown away or used as manure. In this way the 
danger of stains and flaking is much reduced. Bates may be 
mended with fresh portions of dung-infusion for several successive 
packs of hides, but should not be used too long, as they gain in 
solvent power by the dissolved hide-substance and the increased 
fermentation, and the method is not without risk. 

After bating the hides are usually " worked " (" scudded," 
" fine-haired ") on the beam to remove dirt and grease, but in 
America a wash in the wash-wheel is often considered sufficient. 
Goods are occasionally " stocked " (p. 163) from the bates, but 
this is not to be recommended, as it is likely to drive out much 
of the partially dissolved hide-substance and produce undue 
looseness and loss of weight. 

It is difficult to give any definite marks of sufficient bating 
other than the soft and fallen feel of the hides, which is easily 
recognised by a practised hand. One of the earliest signs of 
commencing overbating is the occurrence of bluish patches or a 
bluish tinge somewhat similar to an iron-stain, which, if slight, 
generally disappears in a few days after the hides are taken into 
the liquors. Hen- and pigeon-dung is probably best kept air- 
dried, though, if very wet, or for convenience for immediate use, 
it may be kept in paste like dog-dung. 

Dog-dung should never be allowed to lie exposed to the air, 
or it putrefies and turns black, the bating ingredients are 
destroyed, and it will not puer the goods, which turn black and 
putrid without softening. Dung should, therefore, be mixed to 

^ This seems to have been first suggested by T. Palmer, Eng. Pat. 13,636, 



DELI MING, BA TING, PUERING, DRENCHING 231 

a paste with water and kept in tanks so as to be but little ex- 
posed to the air, when it will retain its puering properties for a 
long time unaffected. Fresh dung should be allowed to ferment 
for at least a week before use. No accurate statement can be 
made as to the quantities required. Eitner states that i to i| 
pails of dung-paste (say 14 to 20 litres) is sufficient for 200 
medium to large lamb-skins for glove-kid. It should be sufficient 
to make the water quite turbid, but not thick or soupy. For 
lamb-skins a temperature of 18° to 20° C. is suitable, which may 
be raised in very cold weather to 25° C. to allow for cooling. 
The time required is from two hours for the thinnest slink skins, 
to twelve to fourteen hours for strong ones. It is well to use 
wooden, and not iron, utensils for handling the dung, and it 
should be strained through a coarse cloth after diluting with 
water. As has been remarked, it is not desirable to keep the 
skins in constant motion in the puer ; they should be stirred or 
paddled for the first twenty to thirty minutes, and then for ten 
minutes every hour for five or six hours, after which they can be 
allowed to lie for a longer period without injury. Puering is 
sufficient when the skins feel quite soft and flaccid, hanging in 
folds in any direction and allowing the flesh to be scraped off 
with the finger-nail. 

Wood recommends that for the puering of sheep-skins dung 
should be allowed to ferment one month before use, and states 
that it deteriorates if kept over three months. The puering 
products are the result of the successive action of many sorts of 
bacteria, and Wood is of opinion that those actually concerned 
in puering originate from the air, or from the vessels in which 
the dung is stored, and are not present in it when excreted. 
Borgman ^ advises that the dung should be kept in a dry con- 
dition, and only made into a paste between a fortnight and three 
weeks before use by covering in a clean cask with cold water, 
and on the following day mixing to a smooth paste with a clean 
wooden " poss-stick," made from wood free from tannin. The 
cask should then be covered up, and allowed to rest undisturbed 
till required. Clean extract-casks are very suitable for the pur- 
pose if carefully and repeatedly steamed out, and Borgman 
advises that a regular series should be arranged so as to supply 
the dung required, the date of mixing being carefully marked on 
pach cask. Throughout the process the utmost cleanliness should 
be observed, and the casks should be carefully steamed out as 
soon as emptied. Immediately before use the dung-paste 
should be heated by steaming nearly but not quite to boiling 
^ Die Feinleder-Fabrikation, Berlin, 1901, p. 69. 



232 PRINCIPLES OF LEATHER MANUFACTURE 

point/ care being taken to avoid the introduction of condensed 
water containing iron, and the dung thoroughly mixed with a large 
quantity (say loo gallons) of water at 45° to 50° C, allowed to 
settle, and drawn off through a basket, and strained into the puer- 
ing paddle through a second basket lined with coarse open canvas 
(such as is used by plasterers to cover windows while the plaster 
is drying). A further quantity of warm water should be poured 
on the residue in the mixing tub and used for diluting that in 
the paddle to the proper volume. The temperature of the liquor 
may reach 42° C. before the skins are introduced. The liquor 
should be of a light colour, greenish to brownish-yellow ; if 
darker, it indicates decomposition of the dung by improper storing 
or too long fermentation, and will be liable to cause staining and 
injury to the skins. About 33 litres of dry dung is required 
per 100 kilos, of wet skin prepared for puering (33 gallons per 
1000 lb.). Dry dung should be of yellow to brown colour ; dark 
brown or black dung is spoiled and unsuitable for use. Wet 
dung is more difficult to judge, but very dark brown or black 
should be rejected, as well as that with a very strong smell, 
indicating that it has already fermented. Borgman's directions 
bear the stamp of experience and common sense, and the book 
as a whole repays study. 

Borgman recommends that the skins should be warmed by 
paddling for some time in water of about 40° C, to which a 
couple of pails of puer-paste have been added, before bringing 
them into the puer, the temperature of which they should reduce 
to perhaps 38° C. The puered skins should feel silky on the 
grain, and even somewhat slippery, and when pressed between 
the finger and thumb a dark impress should be left, and the 
flesh should be tender and easily scraped off. The requisite 
condition will, however, vary somewhat with the kind of skins, 
and the purpose for which they are intended. After puering, 
the skins may be paddled for half an hour in water of about the 
same temperature as the puer. 

The only attempt of which the writer is aware to give an actual 
mechanical and numerical value to the effect on the skin of 
puering and deliming processes is that of Wood, who, in collabora- 
tion with Dr H. Sand and D.~ J. Law, constructed an instrument 
for measuring the exact compression and elasticity of the skin 
under varying loads. ^ For full details the original papers 
must be consulted, but the following brief description may be 

^ This seems a dangerously high temperature, both for enzymes and 
bacteria. 

^ The " pucrometer." Puering, Bating, etc., p. 85 ; J .S.C.I. , 1913, p. 398. 



D ELI MING, BA TING, P UERING, DRENCHING 233 

given here. The apparatus consists of two discs of about i 
cm.^ area, the upper of which is attached to a long balanced 
arm, which is loaded by means of a sliding weight on the prin- 
ciple of the steelyard, and of which the exact horizontality is 
determined by means of an electric contact device. The lower 
disc rests on a micrometer screw, by which the thickness of the 
skin can be determined when it is pressed against the upper disc 
to such an extent as to exactly balance the load. With small 
loads gelatin is perfectly elastic, returning to its original thickness 
when the load is released, and this is also the case to a large 
extent with skin still swollen with lime, but in the delimed or 
puered skin this resiliency is greatly diminished, while the com- 
pressibility is increased. The apparatus is of considerable value 
as giving a numerical value for the puering, which can be referred 
to and repeated at any time, but the conditions are too complex 
for it to be easy to draw theoretical conclusions from the results. 
The skin consists of a mass of gelatinous fibres, much swollen 
themselves in the limed condition, but with some water between 
them, while when puered the fibres are much less swollen, but with 
a larger proportion of water in the interstices. The fibres them- 
selves, especially when swollen, have also a certain degree of 
rigidity ; and all jellies, like indiarubber, oppose an elastic 
resistance to change of shape. When a piece of skin is compressed 
between two discs all these forces come into play in varying 
proportions, but any real compression of the water itself or of 
the actual skin-substance is quite negligible. The first effect, 
especially on puered skin, is to expel the water from the inter- 
stices between the fibres, but if the pressure is sufficient to over- 
come the osmotic force which produces swelling (see Chapter X.), 
it is also expelled from the fibres themselves, and this required 
pressure will be affected by temperature and by the degree of 
alkalinity of the fibre. If jelly is compressed between the discs 
it will bulge round the edges, and the degree to which this occurs 
will of course depend on the relative size and thickness of the 
compressed jelly. The time-element must also be considered, 
especially as regards resilience, as most of these effects depend 
on the flow of water through the skin, where it meets with great 
frictional resistance, and the comparatively small forces tending 
to restore equilibrium must necessarily take time to act. Rapid 
resilience probably depends mostly on the mere deformation of 
shape of the jelly-fibres or of the mass of jelly. 



CHAPTER XV 

PICKLING AND DEPICKLING 

The process of pickling, though long used for the preservation of 
untanned pelts, has become of increased importance from its 
extensive use as a preparation for chrom^e tanning, and much 
light has been thrown on its principles by the researches of Procter 
and Wilson, so that it seems to demand a short chapter to itself. 

The general process consists in the swelling of pelt with an acid, 
and then treating it with a strong solution of common salt, in 
which the swelling entirely disappears, and the pelt becomes very 
flat and thin, and is in fact converted into a sort of white leather, 
which, however, at once swells again if placed in water. The 
principles of the process are adequately explained in Chapter X., 
and need not be here repeated ; but it is highly probable that in 
addition to the disappearance of the swelling osmotic pressure 
there described, an actual " salting out " by the very strong 
salt-solution occurs, which has not been completely explained, 
but which may be either due to the consumption of the solvent 
in hydrating the unionised salt, or to the jelly becoming im- 
permeable to the solution of the unionised salt, and so ex- 
periencing the whole instead of only a small fraction of the outside 
osmotic pressure. There is little doubt that this is the explana- 
tion of the powerful effect of anhydrous alcohol mentioned on 
p. 586, and there are neutral salts, especially ammonium sulphate, 
which will salt out neutral gelatin and pickle skin, with regard to 
which ionisation can hardly come in as an explanation. 

The earlier mode of pickling was to swell (and incidentally to 
" tuck up ") the skins in srdphuric acid much diluted and then 
to bring them down in strong salt solution, but it is now universal 
to add some salt also to the " rising solution " itself to control 
undue swehing. A suitable strength for the rising solution is 
about 80 grm. common salt and 7-5 grm. sulphuric acid per 
litre.^ One hundred c.c. of this solution will require about 15 
c.c. of N/i alkali to neutralise it, and it should be tested after 
each lot of skins, which should weigh when wet about 660 grm. 
per litre of solution, and maintained at the same strength by 

1 This is at the rate of 80 lb. of salt and 7-5 lb. of sulphuric acid per 
100 gallons, or the same number of ounces per cubic foot {cp. p. 581). 

234 



PICKLING AND DEPICKLING 235 

suitable additions of acid. The bath should fall during use to a 
strength requiring about 7-5 c.c. of N/i alkali for neutralisation 
of 100 c.c, and, if necessary, the strength of the original acid 
should be diminished, or the time in the bath should be adjusted 
so that this occurs. 

A much weaker solution of acid than the above is sufficient for 
adequate pickling, and a more satisfactory pelt results. The 
writer once experimented with one-tenth of this acid, and the 
skins kept perfectly, but a longer time in the bath is required, 
which is inconvenient commercially. The acid absorbed by the 
skins is mainly hydrochloric, sodium sulphate accumulating in 
the bath. The salt is not absorbed by the skins in the same way 
as the acid, but will be continually diluted by the water they 
bring in, and occasional additions of salt must therefore be 
made, the density being maintained at about 65° Bkr. (1-065 
sp. gr.). After paddling or being stirred in this bath for about 
I or f hour the skins are transferred to saturated brine, and 
stirred in it till fully fallen in thickness, the density of the liquid 
being maintained by excess of salt. They may be allowed to 
remain some hours in the saturated brine with advantage. 

Within moderate limits the strength of the rising liquor is 
not of great importance, since the skins will only absorb a certain 
amount of acid (increasing with the concentration of salt). In 
the second or falling liquor the large excess of salt forces all the 
acid present into the skins, none diffusing into the bath. Skins 
may be effectively pickled with very much smaller quantities of 
acid than those prescribed above or ordinarily used, and are 
much easier to tan satisfactorily, but it is said that they are more 
liable to suffer from mildew.^ Pickling may also be done by 
placing the skins in a concentrated brine-bath and adding a 

^ Hydrochloric acid in equivalent proportions may be substituted for 
sulphuric, and has been said to prevent mildew and other fungoid trouble, 
which sometimes occur with pickled skins, but both this and the state- 
ment that skins pickled with little acid are more liable to mildew is open 
to much doubt, as tanners are prone to attribute to any new method 
troubles which are really due to faulty manipulation or some accidental 
cause. Sulphuric acid and salt are generally cheaper than hydrochloric 
acid, and in either case it is hydrochloric acid which is absorbed by the 
skin, but if the former mixture is used, and skins are not thoroughly freed 
from lime by previous deliming, a certain amount of calcium sulphate 
may be precipitated in the skins, and this has been shown in the case of 
salt-stains (p. 37) to favour some sorts of bacterial activity. Many of the 
troubles which occur in pickling are not due to the particular acid used, 
but to its careless use, and in particular to the rough way in which it is 
generally measured. 



236 PRINCIPLES OF LEATHER MANUFACTURE 

calculated quantity of acid, not exceeding about i grm. -molecule 
of sulphuric acid per kilo, of dry hide-substance, but the method 
is not economical in practice, from the dilution of the bath pro- 
duced by the water brought in by the skins and the necessity of 
constant large additions of salt. 

Pickled skins must not be brought into contact with water, 
which, by diluting the brine they contain, allows the excess of 
acid to act upon and destroy the fibre. Even drops of water 
accidentally sprinkled on the skins produce this effect, and it is 
said that it spreads to parts which have not been wet. For 
similar reasons it is necessary in- tanning pickled skins at least 
to begin the process in liquors to which salt has been added, the 
quantity required being dependent on the amount of acid used 
in pickling the skins, and where this is reduced to a minimum, it 
is even possible to tan without further addition of salt than that 
contained in the skins. 

In place of mineral acids, organic acids such as formic and 
acetic might be substituted with great advantage as regards 
safety and easy subsequent tannage, but would be much more 
expensive, but in some cases they certainly deserve attention. 
Formic acid especially has itself strong germicidal properties. 
Some years ago Mr A. Seymour- Jones pickled sheep pelts with 
formic acid and salt, and sent them in a light box up the Amazon 
to Manaos and back, and they arrived again in England in a 
perfect condition, spite of the fact that the voyage was through 
one of the most trying regions of the Tropics. ^ The Seymour- 
Jones process for sterilisation of skins against anthrax is also a 
formic acid pickling, and the addition of the small percentage 
of mercuric chloride practically guarantees not only against 
anthrax, but against moulds and mildew. To sterilise dried 
hides they are soaked for twenty-four hours in a solution of 
I per cent, of commercial 90 per cent, formic acid and 0-0002 
per cent, of mercuric chloride, and for skins a less concentration 
of formic acid has proved efficient. For pickling the actual con- 
centration is not so important as that the acid absorbed should 
be about 4-5 per cent, on the actual dry hide-substance of the 
skin. After the acid treatment the skins or hides are treated - 
with saturated salt solution as usual, or even salted with dry salt. 

1 Colleg., 3, 1904, p. 186. Seymour-Jones recommends for skins after 
deliming and drenching a treatment for twenty-four hours in a 0-25 per 
cent, solution of formic or a i or 2 per cent, solution of " pyroligneous " 
(crude acetic) acid free from iron in a paddle which should be run for a 
few hours at first. They should also remain twenty-four hours in the 
saturated salt solution T 



PICKLING AND DEPICKLING 237 

The writer found by analysis of skin treated in this way, that 
after the salting the formic acid was practically entirely replaced 
in the skin by hydrochloric. It may seem rather strange at 
first sight that the weaker formic acid in the hide should have 
been able to liberate and take up the hydrochloric from the salt, 
but the result follows from the mass-law. In treating with a 
very large excess of salt the CI ions are in enormous excess, and 
the result is the formation of a very minute quantity of gelatin 
formate and a comparatively large quantity of gelatin chloride, 
and a similar distribution of the sodium salts. ^ It is therefore 
probable that the process might be cheapened without dis- 
advantage by employing about 3 per cent, of HCl and only 
I per cent, of formic acid on the actual dry material of the skin. 
This would correspond to, say, 2 per cent, of ordinary commercial 
hydrochloric acid on the wet weight of pelt, and it might be 
advisable to add, say, 0-2 per cent, of formic acid to the final salt 
solution rather than mix it with the HCl of the rising solution. 

Several methods of pickling have been suggested which do not 
involve the use of acids. Meunier proposes to treat with a dilute 
solution of bleaching powder, and also with saturated solution of 
potassium carbonate {cp. p. 575), but both of these methods have 
disadvantages which tell against their practical use. It has also 
been mentioned that skins could be dehydrated and preserved 
by soaking in saturated solutions of ammonium sulphate, but 
the ordinary commercial article contains phenolic or tarry im- 
purities, which produce an actual tannage of the skin, and prevent 
its returning to pelt on soaking {cp. p. 576). 

Skins pickled with acids, and especially with the stronger acids, 
still retain enough to cause them to swell excessively if soaked in 
water so as to remove the salt, and if tanned in this state produce 
with vegetable tans a quite rotten leather. The usual remedy is 
to add sufficient salt to the early liquors to prevent swelling until 
the acid of the skins has been sufficiently displaced by tannin, 
and in America it is not unusual not only to add salt but actually 
sulphuric acid to sumach liquors in the first stages of tanning, 
which produces leather with a very much smaller expenditure of 
sumach than in the normal process. The leather is apparently 
fully tanned, and does not appear to contain free sulphuric acid, 
which is probably expelled by the tannin in the later stages of 
the process. 

When it is desired to give a normal tannage without salt the 
skin must be first " depickled," that is, the acid must be neutrahsed 
or removed. This may be done by any weak alkaline solution, 
^ Colleg., II, 191 2, p. 687. 



238 PRINCIPLES OF LEATHER MANUFACTURE 

but considerable care is required not to bring the skin into a 
really alkaline condition. The prettiest process, theoretically, is 
to drum or paddle with an insoluble alkaline carbonate, such as 
" whitening," or with magnesia, but it has the evil in practice 
that some of the whitening is apt to adhere mechanically to the 
skin and produce stains or bad colour. Sodium bicarbonate or 
borax are suitable salts to use, but excess must be avoided, and 
ordinary soda crystals may be used satisfactorily with sufficient 
care. No doubt Stiasny's neutralising mixture (p. 272) of soda 
crystals and ammonium sulphate or chloride in about equal 
quantities would also be safe and efficient. One of the cheapest 
and most satisfactory depicklers is sodium thiosulphate (" hypo "), 
as recommended by Mr Seymour- Jones,^ as it is quite neutral, and 
cannot do injury by excess. 

When pickling is used as a preliminary to chrome tannage, the 
object is quite different from that of its use as a preservative, 
and is generaUy to introduce acid into the pelt. Thus if a heavily 
pickled pelt be introduced into a neutral bichromate solution it 
will be depickled by the action of the bichromate, and sufficient 
chromic acid will be liberated to chrome the skin, and in any 
case a much smaller amount of acid will be needed in the chroming 

1 Colleg., II, 1912, p. 620. Seymour-Jones uses 4 to 5 lb. per dozen 
pelts. He also mentions a variety of other uses to which thiosulphate can 
be put in leather manufacture. Finely ground hypo can be used in place 
of salt as a preservative of hides and skins. It is a powerful dehydrant, 
and if prepared pelt is saturated with hypo and then treated with a weak 
acid, a beautiful white leather results, partly by dehydration, and partly 
from the finely precipitated sulphur, which can be fat-liquored and dyed, 
or, treated with suitable oils, will produce a sort of chamois-leather. 
Instead of an acid, alum or aluminium sulphate can be used to decom- 
pose the hypo and give an alumina tannage, and if formaldehyde be 
further added, a good bufE-leather results. A curious and unexplained 
effect is the removal of grease and 'fat from pelt or leather. If greasy 
hides or skins are delimed by saturated hypo solution and treated with a 
weak acid bath, or cleansed skins are slightly swollen with acid and then 
treated with hypo, in either case in a paddle or drum, the grease can be 
worked out on the beam, or the skins may be chromed without the grease 
subsequently appearing. 

Greasy butts after rolling may be suspended in a 10 to 20 per cent, 
solution of hypo at 140° Fahr. for a few hours, when the grease will 
float to the surface, and should be skimmed off before the butts are 
withdrawn. If subsequently treated with a weak acid, preferably formic, 
the colour will be much improved, but a good deal of tan will be stripped, 
and if used for sole the leather will require re-tannage. Hypo can also be 
used as a bleaching agent for extracts, and if hides are treated with from 
2\ to 10 per cent, of hypo they may be drum-tanned with neat extract in 
a very short time. 



PICKLING AND DEPICKLING 239 

bath than with unpickled skins (see p. 261). For the basic 
process pickling is also often useful, as enabling a much more 
basic liquor to be used at the outset without precipitation or 
" case-hardening " the skin, so that a heavier tannage can be 
given to the interior without overtanning the outside. 

For these purposes it is obviously unnecessary to treat with a 
saturated salt-bath after " rising," and it will generally be better 
to add such an amount of salt to the rising liquor as will prevent 
more swelling than is desired. Instead of free acid, alum or 
sulphate of alumina is often used with or without salt, giving a 
certain amount of preliminary alumina tannage. 

Seeing that the objects of this preliminary pickling vary so 
much, it is impossible to give definite quantities or directions, as it 
really forms a part of the subsequent tanning process, to which it 
must be adapted. 



CHAPTER XVI 

ALUM TANNAGE OR TAWING 

We have now followed the raw material up to the final stage of 
preparation for its actual conversion into leather, and it remains 
to consider the means by which that important change is pro- 
duced. Though as yet the vegetable tanning process is most 
largely used, and possesses the greatest commercial importance, 
the use of mineral salts has long been known, and, through the 
advent of chrome tanning, has placed the permanent supremacy 
of the vegetable tannins in considerable doubt. Not only the 
importance of mineral tanning processes, but their greater sim- 
plicity from the scientific side, justifies their consideration before 
those of vegetable origin. 

In the previous chapters it has been shown that to produce a 
permanent leather it is not only necessary to dry the fibres in a 
separate and non-adherent condition, but so to coat them or alter 
their chemical character that they are no longer capable of being 
swelled and rendered sticky by water. All salts (see p. 123) 
which produce a contraction or dehydration of the fibre similar 
to that caused by alcohol are capable of the first effect in a greater 
or less degree. Many sulphates, and particularly those of sodium 
and magnesiuni, though they will not alone produce leather, will 
so far contract the fibres as to greatly hasten tanning by vegetable 
tanning materials, and they are therefore capable of useful 
application in quick-tanning processes, especially where tough and 
light-weighing leathers are aimed at, which may be subsequently 
weighted and solidified by further treatment. Strong solutions 
of ammonium sulphate are almost as strongly dehydrating as 
alcohol, and will produce white leathers very similar to those 
formed by pickling, a fact which is certainly of considerable 
commercial importance. None of these salts, however, can form 
a complete leather in themselves, but require the assistance of 
metallic salts, which will permanently fix themselves in the fibre, 
and diminish or destroy its attraction for water. Many substances 
have this power in a greater or less degree, but all those of com- 
mercial importance belong to the group of which aluminium, 
iron, and chromium are representative, and which are capable of 
producing salt-forming oxides of the formula M2O3 [e.g. alumina, 

24Q 



ALUM TANNAGE OR TAWING 241 

AI2O3J. Manganese, of which the salts of this type are very 
unstable, has very slight tanning power, while titanium, which 
in many ways is allied to the group, though it does not strictly 
belong to it, has recently been patented as a tanning agent. 
For the present, however, we may limit our attention to the three 
metals first nataed. 

Alumina and its salts demand the first attention, -not only as 
having been used for leather manufacture in very early times, but 
as being still important commercially. The metal aluminium is 
now well known, and its oxide, alumina, AlgOg, is abundant in 
Nature, combined with silica in the form of clay and bauxite, as 
fluoride in combination with sodium fluoride in cryolite, and 
in some cases as a native sulphate. Alum-shale, which was 
formerly the principal source of alum, is a bituminous clay con- 
taining much iron sulphide, and which when calcined yields 
aluminium sulphate. As aluminium sulphate does not crystallise 
readily, and was difficult to free from iron, potassium sulphate 
was added to the liquor obtained by leaching the calcined shale, 
from which, after concentration by boiling, a double sulphate of 
potassium and aluminium, Al2(S04)3,K2S04,24Aq, potash-alum, 
was easily crystallised out. Alum is now usually made by decom- 
posing clay or bauxite with sulphuric acid, and ammonium 
sulphate is generally substituted for the potassium salt, yielding 
ammonia-alum, a double sulphate of aluminium and ammonium 
of similar constitution to potash-alum. Ammonium alum is 
easily distinguished from the potassium salt by the strong smell 
of ammonia which it evolves on the addition of caustic soda or 
lime. So far as is known there is no practical difference in 
tanning effect between the two salts, and ammonium alum is 
cheaper, and slightly stronger, its molecular weight being 906, as 
against 948 for the potassium salt. Either alum dissolves readily 
in cold water to the extent of about 9 parts in 100 of water, 
and more easily, and to a much larger extent, in hot water, from 
which the excess crystallises on cooling. It is said that for 
purposes of leather manufacture alum solutions should not be 
boiled, and, though it is improbable that this produces any 
considerable change, it must be remembered that chrome alum 
on boiling really does undergo decomposition to free acid and a 
more basic salt, indicated by change of colour from violet to 
green, from which it slowly returns to the violet form on cooling. 

Alums are only valuable in leather manufacture in proportion 
to the aluminium sulphate which they contain, the potassium or 
ammonium sulphate taking no part in the reaction except as a 
" neutral salt " (p. 123), and since improved methods have 

16 



242 PRINCIPLES OF LEATHER MANUFACTURE 

rendered possible the production of aluminium sulphate practi- 
cally free from iron, it has largely taken the place of alum, than 
which it is both cheaper and stronger. Crystallised aluminium 
sulphate, Al2(S04)3,i8Aq, has a molecular weight of 666, and is 
of equal value to go6 of ammonia-alum and 948 of potash-alum. 
Iron is the most objectionable impurity in both alums and 
aluminium sulphate, and may be detected by the addition of 
potassium thiocyanate, which will produce a red colour, or 
potassium ferrocyanide ("yellow prussiate of potash"), which 
will produce a blue. As the iron may be present in the ferrous 
condition, it is safer to add a little ferricyanide (" red prussiate "), 
or first to boil the alum solution with a few drops of nitric acid 
or bromine water. For more accurate determination of iron see 
L.I.L.B., pp. 20, 136. 

No satisfactory leather can be produced with a solution of 
alum or aluminium sulphate alone, the skin drying horny, and 
incapable of softening by stretching. In practice, salt is always 
used in addition, the proportion being very variable, but averaging 
about half the weight of alum, or two-thirds the weight of sul- 
phate of alumina employed. The mode of action of the salt has 
long puzzled chemists, and it has been supposed that its use 
was to convert the aluminium sulphate into chloride, a reaction 
which takes place to some extent, but which fails to explain the 
production of a soft leather, since aluminium chloride, though 
freely taken up by the skin, produces alone no more satisfactory 
leather than aluminium sulphate. The real explanation is found 
in Chapter X. Alumina is a weak base, which readily gives up 
its acid to the pelt, becoming converted into a basic salt (see 
below). The acid not only swells the pelt and renders it in- 
capable of producing a soft leather, but the swollen pelt is less 
ready to absorb the alumina salt, and so remains undertanned. 
The addition of salt prevents the swelling effect of the acid, and 
produces a partial pickling of the skin (p. 234), which, in con- 
junction with the tanning effect of the basic alumina salt formed, 
yields a satisfactory leather, though one which is readily affected 
by washing. If instead of adding common salt to the alum 
solution, an alkali such as soda is added, it combines with a 
portion of the acid, forming sodium sulphate, while the alumina 
remains in solution as a " basic salt." As the term " basic salt " 
must be frequently employed in connection with mineral tannage, 
it may here be explained. Basic salts are compounds inter- 
mediate between the normal salt, in which the whole of the base 
is combined with acid, and the hydrated oxide, in which the 
whole is combined with OH groups. Thus aluminium chloride. 



ALUM TANNAGE OR TAWING 243 

AlgClg, is a normal salt, in which the whole of the three combining 
powers of the aluminium are saturated with chlorine ; aluminium 
hydrate, Al2(OH)6, is the hydrated oxide, and AlgClgOH, 
Al2Cl4(OH)2, and so on are basic salts, in which successively more 
of the CI is substituted by OH. It is somewhat doubtful, how- 
ever, whether most basic salts are really definite compounds, and 
not rather, colloid solutions of the hydrate in the normal 'salt. 
Generally, as a salt becomes more basic its solution in water 
becomes more unstable, and very basic salts are either insoluble, 
or are precipitated from their solutions by very trifling causes, 
such as boiling, dilution, or the attraction of animal or vegetable 
fibres, separating into free acid and either hydrate or a still 
more basic and insoluble salt. On this property depends their 
importance in tanning and dyeing, many of the metallic mordants 
being solutions of basic salts. Basic salt solutions are formed 
in various ways, the most common being the direct solution of 
a hydrated oxide in a solution of the normal salt, or the neutralisa- 
tion of a part of the acid of the normal salt by the addition of a 
stronger base. The latter takes place on the addition of soda 
to an alum solution. If the soda is added in excess, the whole 
of the alumina is precipitated as hydrate, or as an insoluble 
basic salt, but if a proportion not exceeding about four parts^ 
of crystallised sodium carbonate be dissolved separately, and 
added slowly with constant stirring to the ten parts of alum dis- 
solved in water, no precipitation wiU take place. In this solution 
leather can be tanned, either with or without addition of salt, the 
alumina is taken up more freely than from the normal alum, and 
the leather is more easily softened, and more resistant to water. 
In fact such leather bears a strong resemblance to the chrome 
tannages, standing a great deal of washing and considerable 
temperatures without returning to a pelty condition. The more 
basic the solution that is used, the fuller and softer is the leather 
produced. The alumina-salt taken up by the skin from such 
basic solutions is always basic, while that absorbed from alum 
or alumina sulphate is apparently the normal aluminium 
sulphate. It is probable, however, that the actual tanning 
salt is in both cases basic, and that the acid is fixed as free 
acid, as in the pickling process, as the proportions of acid 
and base found in the residual liquor are somewhat variable. 
The basic salts will be more fully discussed in relation to 
chromium and iron in Chapter XVII., where they are of greater 
importance. 

Basic alumina solutions have hardly taken the place in practice 
which they deserve, though they were described by Knapp in 



244 PRINCIPLES OF LEATHER MANUFACTURE 

1858 ^ and have since been patented by Hunt, but the patent 
(probably invahd) has long lapsed. A good stock solution for 
practical use is made by dissolving 10 lb. of sulphate of alumina 
in 10 gallons of water, and 4 lb. of washing soda in 4 gallons, 
and gradually mixing the latter with the former. Salt can be 
used in addition if desired, and flour and egg-yolk may also be 
added. Basic alumina solutions can also be used in considerable 
quantities in conjunction with chrome, without rendering the 
leather incapable of standing the boiling test, or materially 
altering its colour. 

In curing small skins, where it is not desirable for the fur to 
come in contact with the liquid, or in the tawing of wool rugs, it 
is often convenient, after freeing the skin as much as possible 
from blood and dirt and adhering flesh, to stretch it on a frame, 
or nail it out on a board, and apply a strong alum-and-salt 
solution, as hot as the hand wiU bear, with a sponge, repeating 
the operation till the skin is struck through. ^ About i lb. of 
alum and | lb. of salt per gallon is a suitable strength. In place 
of applying the solution, powdered alum and salt is sometimes 
rubbed into the wet skin. Alumed goods should generally be 
dried out rapidly, and finally at a good temperature, as this tends 
to fix the tannage, which is also made more permanent and 
resistant to water by keeping the skins for a month or more in 
the alumed condition, an operation known as " ageing." When 
first dried; alumed goods are invariably stiff and horny, and, to 
give them softness, must first be damped back to a flexible 
condition (best by placing in slightly damp sawdust), and then 
gradually softened by mechanical means. " Staking " and 
" perching " are the usual methods, the first consisting in draw- 
ing the goods vigorously over a bluntish blade fixed on the top 
of a post, and the second in fixing the skins on a horizontal 
pole (the " perch "), and working them with the " crutch stake," 
a tool formed somewhat like a small shovel with a semicircular 
blade, in place of which a " moon-knife " (a round blade some- 
what like a broad thin quoit) is often fixed in a wooden crutch. 
The tools and mode of using them are shown in figs. 44 and 45.^ 

^ Natur und Wesen der Gerberei, Braunschweig, 1858. 

^ A similar method can be used with strong basic chrome solutions, 
producing a leather which can be washed, and if the solution be saturated 
'with salt, and allowed to dry on the skin before washing, the crystallisa- 
tion of the salt so softens the skin as to render staking almost unnecessary. 

3 The process shown in fig. ^5 is not actually " perching," but " ground- 
ing," in which a moon-knife with a sharp turned edge is used to reduce 
the thickness of the skin on the perch, at the same time as it stretches 
and softens it, but the action is much the same. 



ALUM TANNAGE OR TAWING 



245 



Machines, described on p. 248, are now generally used for these 
operations. After the first staking or softening the skins are 
allowed to become nearly dry, and are then staked a second time. 
Some judgment is required as to the precise degree of moisture 
in each case : in the first instance the skins must be sufficiently 
damp to yield without injury to the mechanical stretching, but 
in this state they retain sufficient moisture to enable the fibres 




Fig. 4.1 . — Staking White Leather. 



again to adhere on drying ; and at the second staking or perch- 
ing they must be damp enough to allow these fibres to be 
loosened without violence, and dry enough to prevent their again 
adhering. The first stretching must not be too severe. 

The following slight sketch of the manufacture of calf-kid 
will serve to illustrate the practical manufacture of the finer 
alumed or " white " leathers, though little if any is now made. 
The raw material is in England mostly large market-calf, though 
salted and dried skins are sometimes employed. After sufficient 
soaking or washing in water they are limed without arsenic or 
other sulphides, in limes which must not be allowed to grow stale 
or putrid, until the hair can be easily removed. After unhairing 
and fleshing in the usual way they receive a few days in a pretty 



246 PRINCIPLES OF LEATHER MANUFACTURE 

fresh lime in order to plump them, and are then freed from lime 
gradually, but as completely as possible, by successive steepings 
and washings in water softened by a mixture of that already 




Fig. 45. — Grounding with the Moon-knife. 



used on other goods and by working on the beam. This acts as 
a partial substitute for puering with dung, which is no longer 
used on calf-kid. Probably an oropon or pancreol bate is now 
employed. The goods are next drenched in the ordinary way, 



ALUM TANNAGE OR TAWING 247 

3 to 4 per cent, of bran being used, and the goods allowed to rise 
two or three times in the drench, which should be conducted 
with the usual precautions (p. 215) to avoid the danger of false 
fermentation in hot weather. The goods should come out of the 
drench free from lime and unswollen by acid, but full, white, 
and soft. The tanning (or " tawing " as it is usually called in 
the case of alumed goods) is done in a rotating drum with a 
mixture of alum or sulphate of alumina, salt, flour, egg-yolk, and 
olive oil. About 5 per cent, of flour, 2 5 per cent, of alum, i per 
cent, of salt, the yolks of 25 eggs or i| lb. of preserved egg-yolk, 
2 oz. of olive oil, and i-^ to ij gallon (12 to 15 lb.) of water are 
required per 100 lb. of wet pelt. The flour is first made into a 
smooth paste with a little water, the egg-yolk, somewhat diluted 
with warm water and strained, is mixed in together with the oil, 
and finally the alum and salt solution is added at such a tempera- 
ture as to bring the whole mixture to blood-heat (38° C). The 
length of drumming depends on the thickness of the skins, several 
hours being required for very thick ones, but care must be taken 
to stop and ventilate the drum at frequent intervals, so as to 
prevent the skins becoming hot by friction. This part of the 
process was formerly accomplished by treading with bare feet 
in a tub. After tawing the goods are allowed to lie in piles over- 
night, or are sometimes laid in tanks for a day or so with any 
that remains of the tawing paste, to complete the absorption of 
the salt and alum, and are then frequently split with the band- 
knife machine, though it would be better, as is often done on the 
Continent, to split them before tawing, the materials of which 
are not only costly, but unfit the splits for many purposes for 
which they might be employed. The drying should be rapid, 
but is best done first at a moderate temperature, or in the open 
air, and then in a rather hot stove. They may now be allowed 
to " age " from one to three months, but it is usually better 
before ageing to do the first part of the finishing process, con- 
sisting of damping back, staking, and, if necessary, shaving. 
Machines are now almost invariably used for the staking, the 
principle of which may be described as that of a pair of tongs, 
carrying one or generally two staking blades on one limb, and a 
roller on the other which closes on the skin, and presses it against 
and between the blades, while the tongs are drawn backwards, 
aUowing it to slip through. Fig. 46 illustrates the Slocomb, one 
of the most popular machines of this type. After staking and 
ageing the skin is soaked in water till thoroughly wet in all parts. 
This not only softens it and prepares it for dyeing, but takes out 
the superfluous alum and salt, and at the same time a good deal 



248 PRINCIPLES OF LEATHER MANUFACTURE 



of flour and egg. To replace these " re-egging " is necessary, 
and while some manufacturers give egg-yolk, or egg-yolk and 




flour only, many add a proportion of salt, and sometimes also of 
alum. This is done before dyeing if the skins are to be blacked 
on the table, but as tcay-dyeing (see p. 499) would again wash out 



ALUM TANNAGE OR TAWING 249 

the egg, the re-egging is deferred till after dyeing if this process is 
resorted to. Before dyeing the skins receive an alkaline mordant 
to overcome greasiness and enable them better to take the 
colour. In former times this was usually stale urine, but this 
has mostly been superseded by solutions of " hydro! eine " (a 
washing powder), or of soap rendered more or less alkaline with 
ammonia. Eitner gives the following recipe, viz. ^ lb. Marseilles 
(olive oil) soap dissolved in boiling water, 5 or 6 egg-yolks added, 
and the whole made up to 4 gallons with water and J lb. potash 
bichromate. The colour used is infusion of logwood or its 
extract, or two-thirds logwood and one-third fustic, ^ which is 
best extracted without alkali, a small quantity of soda or ammonia 
being afterwards added. Coal-tar dyes are often added. It is 
fixed and darkened by a wash of iron-liquor or a solution of i of 
ferrous sulphate in 75 of cold water. After being again dried 
the skins are sometimes grounded with the moon-knife, softened 
again by staking or perching, for which a machine with inclined 
or spiral blades attached to a drum and working on a sort of 
leather apron is often preferred to machines of the Slocomb 
type, and rubbed over on the grain with a composition containing 
oil, wax, etc., and are finally ironed with a heavy fiat iron to 
give them a fine and smooth surface. Eitner gives a recipe for 
the gloss : i kilo, gum arable, J kilo, yellow wax, | kilo, beef- 
tallow, f kilo. Marseilles soap, i litre strong logwood infusion, 
and 5 litres water. The water is brought to a boil in an earthen 
pot, and then the soap, wax, gum, and tallow are added suc- 
cessively, each being stirred till dissolved before adding the next, 
and lastly the logwood. After boiling for an hour it is allowed 
to completely cool, being incessantly stirred during the whole 
process. After ironing the goods are rubbed over with a final 
gloss, for which Eitner gives the following recipe : 8 litres ohve 
oil, 500 grm. tallow, 500 grm. yellow wax, 500 grm. rosin, 
500 grm. gum arable (previously softened in water). The 
mixture is cooked for two hours in an earthen pot till the water 
is evaporated, and allowed to cool with constant stirring. The 
skins are then rubbed with a flannel with a very smaU sprinkling 
of French chalk, and are ready for sale. 

The manufacture of calf-kid has been almost entirely super- 
seded by that of box-calf, from the superior water-resisting powers 
of the latter, and also on account of the costly nature of the 
materials employed, one Leeds manufacturer in former times 
using not less than 50 tons of egg-yolk annually, but the leather 
made a very smart and comfortable boot, and a chrome calf-kid 

^ The addition of fustic is to correct the blue-black of the logwood. 



250 PRINCIPLES OF LEATHER MANUFACTURE 

might very probably come into fashion again. The difficulties 
in its manufacture are, however, considerable, as it is almost 
impossible with basic chrome liquors to get the absolute smooth- 
ness of grain required, and for this cause chrome leathers have 
usually a boarded grain. It is possible that if the tawing were 
commenced with an alum tawing paste, and a suitable basic 
chrome liquor only added when the paste were almost absorbed, 
this difficulty might be overcome. Earlier substitutes for egg- 
yolk were unsuccessful, either because unsatisfactory in effect 
or equally costly with the genuine article, but with our largely 
increased knowledge of emulsions this difficulty might probably 
be overcome, perhaps by the use of sulphonated oils. The flour, 
of which the gluten only, and not the starch, is absorbed, and the 
proteids of the egg-yolk, which are important as fillings, might 
probably be replaced by other colloids of an organic nature, or 
by minerals such as colloidal silica or alumina phosphate. 

The manufacture of glove-kid is quite similar in principle to 
that just described, but varied in detail to suit the softer and more 
delicate skins employed, to give greater softness, and especially 
the quality of stretching in any direction without springing back, 
which is so characteristic of the leather. Lamb-skins are the 
principal raw material, though genuine kid is also employed for 
the best qualities. The manufacture varies much with the 
quality and character of the goods. The skins, which are mostly 
dry, are soaked in clean and cool water for three to four days, 
according to age and thickness. Common qualities (small im- 
ported slink lambs) are often unhaired by dipping in or painting 
with a paste of gas-lime, lime and sulphide of sodium, or lime 
and red arsenic, so as to destroy the wool. Better skins are 
sometimes unhaired by painting on the flesh with lime alone or 
in mixture, and in other cases ordinary lime-pits are used with 
limes, which are most usually strengthened with red arsenic, 
which is added to the lime while hot from slaking [cp. p. 189). 

The calcic sulphydrate (and perhaps sulpharsenite) thus formed 
hastens the unhairing, and preserves the gloss of the grain. 
Well conducted glove-kid establishments avoid as much as 
possible the use of old limes, which produce a loose, porous 
leather, with a rough, dull grain. The liming lasts on the average 
ten days, and is of the greatest importance. It is essential that 
the inter-fibrillary substance should be dissolved, that the leather 
may have the quality known in Germany as Stand, that is to say, 
may be strongly stretched in either length or breadth without 
springing back. It also depends upon the liming (and this is of 
special importance in the case of lamb-skins) whether the tissue 



ALUM TANNAGE OR TAWING 251 

of the fat -glands is well loosened, so that the fat, either as such, 
or as lime- or ammonia-soap, may be readily and completely 
worked out. Skins in which this is neglected can never be 
properly dyed. 

When the hair (or wool) is well loosened, the skins are rinsed 
in water, and then unhaired on the beam with a blunt knife. 
The water employed in washing should not be much colder than 
the limes, or it will prevent the hair from coming away readily. 
The wool or hair is washed and dried for sale. The skins are 
thrown into water,, to which a little lime-liquor has been added, 
to prevent precipitation of the lime in the skins by the free car- 
bonic acid of the water, which would have the effect of making 
them rough -grained. 

Next comes the first fleshing (Vergleichen) or " levehing." 
By this the loose cellular tissue on the flesh side is removed, 
together with the head, ears, and shanks ; and the flanks are 
trimmed. The skins are then again thrown into water softened 
with lime-liquor as above described, and then into a puer of 
dogs'-dung. This is prepared by stirring up white and fermented 
dogs'-dung with hot water, and straining it through a sieve or 
wicker basket. The puer must be used tepid, and not too strong. 
The skins " fall " (lose their plumpness) in it rapidly, and become 
extremely soft and fine to the touch ; and the fat -glands, remain- 
ing hairs, and other dirt can now be very readily scudded out. 
One main object of the puering is to remove the elastic fibres, 
which are very abundant in the grain layer, and prevent stretching. 

Too strong puers, or too long continuance in them, produce 
evident putrefactive effects on the skins (see also p. 231). 

When the skins come out of the puer they are stretched and 
worked on the flesh with a sharp knife, and any remaining sub- 
cutaneous tissue is removed. This constitutes the second flesh- 
ing. They are then rinsed in warm water, and beaten with clubs 
in a tub, or worked in a tumbler-drum, in either case with a very 
little water only ; and are finally brought into a tank of water, 
not too cold, and kept in constant motion with a paddle-wheel. 

The skins are next cleansed on the grain side by working on 
the beam with plates of vulcanite set in wooden handles, so as to 
remove fat, lime- and ammonia-soaps, and other lime compounds, 
together with all remaining hair or wool. The skins are now a 
second time washed in the " paddle-tumbler," first in cold and 
then in tepid water ; and after allowing the water to drain from 
them, they are transferred to the bran-drench. 

This is prepared by soaking wheaten bran in water at about 
50° C, and diluting with warm water. Sometimes the mixture is 



252 PRINCIPLES OF LEATHER MANUFACTURE 

strained and the bran-water only used, to save the trouble and 
cost of removing adhering particles of bran from the delicate 
skins. Sufficient of the liquid must be employed to well cover 
the skins, and the temperature may range from 50° F. (10° C.) 
to 68° F. (20° C). These conditions are favourable to bacterial 
activity, which comes into play, and, on the one hand, evolves 
acetic and lactic acids, which dissolve any remaining traces of 
lime, and, on the other, loosens and differentiates the hide tissue, 
so as to fit it to absorb the tawing solution. Much care is required 
in the management of the bran-drench, especially in summer, 
since the lactic readily passes into some other fermentation (see 
also p. 215). The tawing mixture is composed (like that em- 
ployed in the manufacture of calf-kid, q.v.) of alum, salt, flour and 
egg-yolks, in a quite thin paste. A small quantity of olive oil is 
also generally used. The skins are either trodden in it with the 
feet, or more generally put into a tumbler-drum with it. Kath- 
reiner pointed out, many years ago,i that a mixture of olive oil 
and glycerine might be partially substituted for the egg-yolks 
in both the tanning and dyeing of glove-kid leather. 

The tawed skins are now dried by hanging on poles, grain 
inwards. Rapid drying in well-ventilated, but only moderately 
heated, rooms is essential to the manufacture of a satisfactory 
product. 

The dry leather is rapidly passed through tepid water, and 
after being hung for a very short time, to allow the water to 
drain off, is trodden tightly into chests, and allowed to remain in 
them for about twelve hours, so that the moisture may be uni- 
formly distributed. It is then trodden on hurdles (German 
Harden) composed of square bars of wood, joined corner to corner, 
so as to make a floor of sharply angular ridges. The next opera- 
tion is stretching with the " moon-knife " ; after which the 
leather is dried nearly completely, and staked again. 

This completes the tawing process. The goods are now 
" aged " as in calf-kid manufacture. Before dyeing they are 
washed with tepid water to remove part of the tawing mixture 
and, especially, superfluous alum and salt, and are re-egged much 
like calf-kid, before dyeing if the latter is done by brushing, and 
after if in the dye-tray or paddle. Aniline colours are more 
used than formerly, especially for topping and brightening the 
natural colours, but the dyewoods and other mordant colours 
are still largely employed. The leather is first prepared with 
an alkaline mordant (stale urine, ammonia, etc.) {cp. p. 493), 
then repeatedly brushed with or djpped in the dyewood liquor, 
^ Gerber, i. (1875) p. 170 ; ii. (1876) p. 664. 



ALUM TANNAGE OR TAWING 253 

and a mordant wash (" striker," German Ueberstrich) containing 
some metallic salt is generally applied, with the object either of 
bringing out the special tone required, or of making the colour 
more lively and permanent. The striker is usually a solution of 
one of the so-called " vitriols " : " white vitriol " (zinc sulphate), 
" blue vitriol " (copper sulphate), " green vitriol " (iron sulphate), 
a tin solution (" spirit "), or occasionally other salts. 

After the dyeing the skins, if dipped, are wrung out and 
re-egged ; if brush-dyed, sleeked out with a brass or ebonite 
sleeker to get rid of superfluous water. They are then dried in an 
airy room. Before staking (stretching) the skins are laid or hung 
in a damp cellar or in moist sawdust. They are staked twice : 
once damp, and once nearly dry ; and are finished by glassing 
or ironing. 

Skins which are much damaged on the grain, or otherwise 
faulty, are smoothed with lump pumice on the flesh side, or 
fluffed with fine emery on the fluffing wheel. They are then 
dyed on the flesh side, mostly by dipping, but occasionally with 
the brush, in which case the method described is slightly modified. 

Tawing with alum and salt is frequently employed for commoner 
and stronger leathers, such as aprons (of sheep-skin), leather for 
whip-lashes, laces for belts, and " skivers " for capping druggists' 
bottles. The process is practically the same as for calf-kid, 
except that no egg and little flour is used. Often flour is entirely 
omitted, and the goods may then be alumed in tubs, in which 
they are merely handled, as the alum solution penetrates quickly. 
Goods which are required white are frequently handled or tumbled 
with a milk of " whitening," both to improve the colour and to 
neutralise any acid present, and fix the alum by rendering it 
more basic. Alumed goods can be stuffed with greases, either by 
hand or in the drum, after thorough softening by staking. 

Alum and other salts of alumina are frequently used in com- 
bination-tanning with vegetable materials (see Chapter XXIL). 
" Green " leather for laces, " dongola," and " dog-skin " glove- 
leathers are made in this way. Glazed kid for ladies' shoes 
must be slightly vegetable-tanned on the surface or it will not 
glaze, but this is frequently accomplished by the use of materials 
in the dye-liquor containing tannins. This was long considered 
a French secret. 



CHAPTER XVII 

CHROME AND IRON TANNAGES 

Both chromium and iron, like aluminium, form trivalent salts 
which have strong tanning properties, but in most other respects 
they are very different not only from it, but from each other. 
While the atomic weight of aluminium is only 27, those of 
chromium and iron are 52 and 56 respectively — nearly double as 
much. While aluminium, so far as is known, only forms one oxide, 
AlgOg, iron and chromium form quite a series, chromium being 
the more oxidisable of the two, and while alumina is amphoteric, 
forming not only salts with acids but aluminates with bases, the 
chrome and iron oxides are either acid or basic, but not both.^ 
The salts of Cr" (chromous salts) are blue, but so avid of oxygen 
that they can hardly be preserved, while the ferrous salts are 
green, and only moderately oxidisable. The trivalent salts of 
chromium are green or violet, while those of iron (ferric salts) are 
yellow or orange, and are stable in absence of reducing sub- 
stances. ^ While iron has a very unstable higher oxide, forming 
red ferrate salts, the corresponding oxide . of hexavalent Cr, 
chromic anhydride, CrOg, is deep orange or red, and forms very 
stable chromates and bichromates, and there is yet another 
oxide, probably per chromic acid, which, when chromic acid is 
oxidised with hydrogen peroxide, can be shaken out with ether 
as a bright blue solution. Both ferric salts and chromates are 
reduced by light in presence of organic matter, but not salts 
of CrgOg. These differences are important in many ways with 
regard to their tanning properties. 

Metallic chromium is a grey, and very infusible, metal, derived 
principally from chrome iron ore, a mineral which contains the 
oxides both of chromium and iron. This is furnaced with a 
mixture of lime and soda or potash, when it absorbs oxygen 
from the air, the chromium becoming converted into chromic 
acid, which combines with the alkali present, while the iron 
remains undissolved as ferric oxide. On lixiviating the mass and 

^ Sodium and potassium hydrates dissolve small quantities of chromium 
hydroxide to pink or green solutions, perhaps of sodium chromite. 

^ A curious exception in colour are the iron oxalates, of which the 
ferrous is orange-brown, while the ferric is deep green. 

254 



, CHROME AND IRON TANNAGES 255 

evaporating the solution, lime and potassium or sodium chromates 
are obtained, according to the alkali used, and on adding sufficient 
sulphuric acid to combine with half the base, potassium or sodium 
dichromate (or, as it is commonly called, " bichromate ") can be 
crystallised out. Potassium dichromate is most commonly made, 
because it crystallises well and is not deliquescent, but sodium 
dichromate is somewhat cheaper, though less convenient in 
storage. In making concentrated basic liquors (p. 267) its 
greater solubility is useful. Bichromates, at least in the crystal- 
lised state, -are not hydric salts like bisulphates, but anhydro- 
chromates corresponding to the potassium anhydrosulphate 
obtained by fusing ordinary bisulphate, and to fuming sulphuric 
acid. Thus the formula of potassium dichromate is : 



[CrOpK 
O 
CrOoOK 



^ O , or Cr2K207, 



and its molecular weight is 294, while that of sodium dichromate, 
which is similar in constitution, but crystallises with 2Aq, is 298. 
The molecular weight of CrOg is 100. By the addition of an 
equivalent of alkali to dichromates, yellow normal chromates, as, 
e.g., K2Cr04, are formed. Chromic acid and acidified potassium 
dichromate are powerful oxidising agents, and are used as 
such in many processes, and especially in the manufacture of 
alizarine. If sulphuric acid be used in molecular proportions, 
the product of the reaction is chrome-alum: 4H2S04 + Cr2K207 
=30+40H2 + K2Cr2(S04)4. This, like ordinary alum, crystal- 
lises with 24Aq, and hence has a molecular weight of 998. It 
forms dark puiple, almost black, crystals, which are a fine garnet- 
red by transmitted light. In cold water it dissolves to a violet 
solution, which becomes green on boiling, but very slowly re- 
sumes the violet condition when cold. This change, which is 
not uncommon in chrome solutions, is probably due to a partial 
decomposition into free acid and a basic salt, the basic salts of 
chromium being generally green. It has been noticed that raw 
pelt sweUs much more in the green than in the violet solution, 
and the violet solutions tan more rapidly.^ Being derived from 
waste products, chrome-alum is often a cheap and valuable 
source of chromium for chrome tanning. 

For the analysis of chrome compounds see L.I.L.B., pp. 141 
et seq., and L.C.P.B., p. 120. 

Chrome is not only of importance in tanning, but in dyeing, 
on account of its power of forming insoluble colour-lakes with 
^ Burton and Hey, J.S.L.T.C., 4, 1920, 205, 272. 



256 PRINCIPLES OF LEATHER MANUFACTURE 

many mordant colouring matters. For this purpose normal or 
basic chromic salts are sometimes used, sometimes chromic acid 
or dichromates, the latter acting not only by yielding chrome- 
oxide on reduction, but as oxidising agents to the colouring 
matters. Most of the colours produced with chrome mordants are 
of dark shades, that with logwood being deep violet or black. 
The mordanting power of chromium is important in the dyeing 
of chrome leather. Bichromate of potash is often used in dilute 
solution for darkening the shade of leather dyed with other 
materials, but is not to be recommended on account of its 
destructive action on the leather. 

Numerous patents have been taken for processes of chrome 
tannage. The first practical method was described by Professor 
Knapp in 1858 (see p. 264), though he did not recognise its 
value. Some of the patents have a historical interest, though no 
practical importance. Among these may be mentioned that of 
Cavallin, a Swedish apothecary, whose object was dyeing rather 
than tanning, but who treated raw hide with a solution of 
bichromate, which was afterwards reduced on the fibre by one 
of ferrous sulphate. ^ The leather produced is dark reddish-brown, 
and tender from the amount of basic ferric salt formed at the same 
time. Mr J. "W. Swan, well known in connection with photo- 
graphic processes and electric lighting, also patented a process of 
chrome tannage (as an addendum to a patent on carbon printing), 
in which the chromic acid first fixed in the pelt was reduced by 
" oxalic. Or other suitable acid." Although it is possible to pro- 
duce leather within the lines of the patent, the strongly acid 
reaction of the reducing agent renders it unsuitable for practical 
use. The first chrome-tanning process which made any show of 
practical success was that patented in 1879 by Heinzerling, 
which was acquired in this country by the Eglinton Tanning 
Company, and also worked under their license for a short time 
by the Yorkshire Tanning Company at Leeds. Though the 
process was not commercially successful on any considerable 
scale, it possesses points of interest which make a brief description 
desirable. Th'e hides or skins, after preparation in the usual way, 
were treated in a mixed solution of salt, alum (or aluminium 
sulphate), and potassium bichromate, but no systematic attempt 
was made to reduce the chromic acid to a tanning form, the pro- 
duct being, at first at least, merely an alum tannage, coloured, 
and perhaps somewhat hardened with chromic acid, though on 

^ Both Chad-wick in America (U.S.A. Pat. 561044, 1896) and Gottschalk 
(Fr. Pat. 258228) seem to have re-patented the Cavallin process with slight 
modifications. 



CHROME AND IRON TANNAGES 257 

keeping for a length of time reduction gradually took place at 
the expense of the hide-fibre and of the fats employed in curry- 
ing, so that the leather internally became greyish-green, and 
really chrome tanned. Specimens of the early products of the 
process, preserved in the museum of the Leather Industries 
Department at Leeds, have now all undergone this change, but 
are still tough and flexible, showing that the rapid tendering of 
the Heinzerling leather, which was one of the causes of its failure, 
must have been due to some error in manufacture, and was not 
inherent in the process. Interesting, historically, is the fact that 
at an early stage in the life of the patent a specimen of the 
leather was submitted to the late_ Professor Hummel in order 
that he should suggest some means of overcoming the disagree- 
able yellow colour of the product. He reduced it with a bi- 
sulphite, and coloured it with an aniline dye, and a piece is still 
in the possession of Leeds University, and in perfectly sound 
condition. If legal publication of this experiment could have 
been proved it would have invalidated the important Schultz 
patents, under which most of the chrome-kid of the United States 
was manufactured. As bearing on modern chrome tanning, the 
most important reaction in the process is that of the alum with 
the bichromate. It has been shown by Heal and Procter ^ that 
pelt absorbs practically no chromic acid from bichromate unless 
it has been previously set free by acidification. When, however, 
alum or sulphate of alumina is added, its sulphuric acid liberates 
the chromic acid, leaving a basic alumina salt in solution, and 
this fact has been utilised in some modern tanning processes. 

The first really important advance in practical chrome tanning 
was made by Augustus Schultz in 1884. Schultz was not a 
tanner, but a chemist employed by a New York firm of aniline- 
colour merchants, and his attention was accidentally drawn to 
leather by a friend who asked him if it were possible to produce 
a leather for covering corset steels which would not rust them 
as ordinary alumed leathers do. The process which he adopted 
was probably suggested by a method then recently patented for 
the mordanting of wool by chrome oxide, and depended on the 
power of the pelt to absorb free chromic acid (as it does all other 
free acids), and the subsequent reduction of the latter on the 
fibre to a basic chrome salt, which produced the tannage. The 
reducing substance employed was the free sulphurous or thio- 
sulphuric acid of an acidified solution of sodium thiosulphate 
(hyposulphite), and as it was not certain which of the two acids 
was the really active agent, Schultz duplicated his patent so as 
^ Journ. Soc. Chem. Ind., 1895, p. 251. 

"-7 



258 PRINCIPLES OF LEATHER MANUFACTURE 

to cover both. Though he made no claim in his patent to having 
discovered the best proportions of his ingredients, those which 
he specified have proved practically useful after allowing for the 
modifications required by different skins and slightly different 
methods of working. His first bath consisted of a solution of 
5 per cent, of bichromate of potash and 2| per cent, of con- 
centrated hydrochloric acid (or i 25 per cent, of concentrated 
sulphuric acid), reckoned on the wet weight of the prepared 
pelt, and dissolved in sufficient water for convenient use in the 
paddle or drum which was to be used in the process. In this 
bath the skins were worked till they took a uniform yellow 
colour throughout, but without any tanning effect being pro- 
duced. They were now freed from superfluous chrome liquor by 
draining or " putting out," and transferred to the second bath, 
which consisted of 10 per cent, of " hypo " and 5 per cent, of 
hydrochloric acid similarly dissolved. In this they rapidly took 
a duck-egg green colour from the reduction of the chromic acid ; 
and when this was un form throughout the skin, the tannage was 
complete. The exact quantity of water is not of great importance, 
and good results can be obtained with anything varying from 20 
to 50 gallons per 100 lb. of pelt (200 to 500 per cent.) if time bc' 
allowed for the weaker solution to act. The quantities of " hypo " 
and hydrochloric acid given for the second bath are often some- 
what insufiicient, and have to be slightly increased to complete 
the reduction. The reaction which takes place in the first bath 
is represented by the fc flowing equation, in which the weights 
of the materials taking part in the reaction are also given below 
the symbols. 



Potassium 
dichromate. 


Hydrochloric 
acid. 


Potassium 
chloride. 


Chromic 
acid. 




Water 


K^CrgO, 


+ 2HCI = 


2KCI + 


2Cr03 


+ 


OH2 


294 


+ 73 = 


149 + 


200 


+ 


18 



As ordinary concentrated hydrochloric acid does not contain 
more than about 30 per cent, of actual HCl,i about 2-5 parts 
would be required to completely decompose 2-94 parts of di- 
chromate, while in Schultz's formula only 2-5 parts of hydro- 
. chloric acid are used to 5 parts of dichromate, thus leaving a 
considerable part of the dichromate undecomposed. This excess 
has been found useful in the production of a good leather, both 
to prevent accidents from an overdose of hydrochloric acid, and 

1 Acid of sp. gr. i-i6 (32° Tw.) contains 31-5 per cent, of HCl by weight 
or 36-6 grm. per litre, and therefore is practically 10 x normal strength. 
Acid of sp. gr. 1-2 (40° Tw.) contains 39-1 per cent., or 469 grm. per litre. 



CHROME AND IRON TANNAGES 259 

because of the modifying effect of an excess of neutral salt on 
the action of the chromic acid, but is not essential. 

The reactions which take place in the second bath are some- 
what complicated. Eitner, in a valuable series of articles on 
chrome tannage, which appeared in the " Gerber " during 1900, 
states that even better results are obtained by using the 
hydrochloric acid in slight excess in the first bath, as the action 
of chromic acid (in the presence of the potassium chloride of 
the chrome-bath) is not swelling but hardening to the skin, 
and the slight swelling action of the hydrochloric acid tends to 
counteract this, and also to facilitate the subsequent reduction. 
The two views are not contradictory, as the excess of bichromate 
behaves to the hide as an alkaline salt, which also produces a 
slight swelling effect, and it is quite probable that better results 
are attained when the solution is either alkaline or acid than 
when the potassium chromate is exactly decomposed. Eitner 
recommends the use of 4 parts by weight of bichromate and 4 
parts of the strongest hydrochloric acid dissolved in 400 parts 
of water for each 100 parts of wet pelt, which should 5deld about 
40 parts of dry leather. He states that if such a bath be used, 
it may be safely and economically exhausted by a second pack of 
skins, which is impossible in a bath containing excess of un- 
acidified bichromate. He gives ^ the following explanation of 
the successive changes which take place when acid is gradually 
added during the reduction, but points out that in practice the 
reactions always to some extent go on simultaneously. 

In the first stage of reduction very shght acidification is 
required, and if the skins have been chromed with excess of 
hydrochloric acid, may be altogether dispensed with. The skins 
become brownish from the conversion of the chromic acid into 
so-called " chromium dioxide " (probably really a basic chromic 
chromate, Cr2Cr04(OH)4, which on ignition leaves CrgOg) ; no 
sulphurous acid is liberated or sulphur deposited, but sodium 
tetrathionate is formed in the bath, and the reaction may be 
represented as follows : — 

(i) aCrOg + eHCl+eNa^SA^SNaaS^Oe + eNaCl+sOHa + CrgOe. 
Further addition of hydrochloric acid brightens the colour of 
the skins, while the liquid still remains clear, and chromium 
chloride is formed instead of chromic chromate, the main reaction 
being : 

(2) 2Cr03 + 12HCI + 6Na2S203=3Na2S406 +2CrCl3 
+ 6NaCH-60H2. 

1 Gerber, rgoo, p. 297. See also Stiasny and Das, J .S.C.I. , 19 12, p. 753. 



26o PRINCIPLES OF LEATHER MANUFACTURE 

On still further addition of hydrochloric acid, sulphur is 
separated according to the following equation, and is deposited 
partly in the skins and partly in the bath : 

(3) 2Cr03 + 6HC1 + sNaaSgOg = sNagSO^ + 3S + 2CrCl3 + sOHg. 

After complete reduction and consumption of the free hydro- 
chloric acid, further reactions take place at the expense of the 
excess of thiosulphate which should be present, resulting in the 
production of basic chromic salts, and the further deposition of 
sulphur, mostly within the skin, as shown in the following 
equations : 

(4) Cr2(S04)3+Na2S203 + OH2=2CrOH.S04 + S02 + S+Na2S04. 

(5) 2CrCl3 + Na2S203+OH2=2CrOH.Cl2+S02 + S+2NaCl. ' 

The thiosulphate bath, therefore, not only reduces but precipi- 
tates sulphur in the skin, and reduces the chromic salt to a basic 
state. In boiling solution thiosulphate precipitates the whole of 
the chromium as chromic oxide, but in the cold, and in presence 
of free sulphurous acid, it only reduces to a basic salt. Eitner 
does not consider the possibility, which certainly requires investi- 
gation, that instead of basic salts, sulphite-sulphates are formed, 
at least in the first instance. Such salts of one base and two 
acids are quite possible, and it is known that the presence of 
acids even as weak as carbonic allows much more alkah to be 
added without causing precipitation. 

The free sulphur which is liberated is partially deposited on 
and among the fibres of the leather, and adds to its softness, and 
also acts chemically on the oils used in " fat-hquoring," so that it 
is probably one of the main causes of difference between the 
products of the Schultz or " two-bath " method and the " one- 
bath " processes subsequently to be described. 

It does not fall within the scope of this book to describe in 
detail the working methods for the production of the different 
kinds of chrome leather, but a few precautions common to all 
forms of the process may be named. It is not absolutely im- 
portant in all cases that goods should be completely freed from 
lime before chrome tannage, but in this case a sufficiency of acid 
must be allowed in the first bath to neutralise the lime introduced. 
Fairly thorough liming is generally advisable to plump and 

1 Obviously the proportion of these reactions will vary with the con- 
centration of the solutions and the rate at which acid is added. Stiasny 
states that more tetrathionate is formed if even a trace of arsenic is present 
in the acid used. 



CHROME AND IRON TANNAGES 261 

separate the fibres, but as a rule the bating or puering of goods 
for chroming should not be excessive,^ but should be planned 
not to remove more than is absolutely necessary of the hide- 
substance, as the chrome tannage is in its nature soft and light, 
and does not lend itself to artificial fillings, such as the flour and 
egg-yolk of the calf-kid process. Skins are sometimes freed from 
lime by " pickling " (p. 238), and pickled skins may be chromed 
without depickling, as the acid will be removed by the dichromate, 
but in this case the acid contained in the skins must be considered 
in the composition of the chroming bath. Skins, indeed, which 
are pickled with a sufficiency of acid may be chromed in a neutral 
dichromate bath, and this is sometimes a convenient mode of pro- 
cedure. To prevent drawing of the grain during tanning, skins 
not unfrequently receive a preliminary tannage with alum or 
sulphate of alumina, and these materials, together with salt, may 
be introduced into the chroming bath, in which case they will 
liberate a portion of the chromic acid, as has been mentioned 
in connection with the Heinzerling process. Alum, chrome- 
alum, and acid salts, such as sodium bisulphate, may be sub- 
stituted for the acid in the chrome bath, but organic acids must 
not be used, as they would reduce the chromic acid. The 
quantity of free chromic acid in the chrome bath is of the most 
vital importance to success, as it, and not the dichromate (which 
may be present in considerable excess), regulates the amount 
of chrome taken up by the skin and the subsequent degree of 
tannage. It is very possible to injure leather by over-chroming, 
rendering it rough, harsh, and even tender. If a bath containing 
excess of bichromate is to be re-strengthened, it may be as- 
sumed as a rule that all the free chromic acid has been absorbed 
by the skins, and while it is merely necessary to restore the 
strength of the dichromate to its original amount, the full quantity 
of acid must be used which would be required in preparing a 
new bath. Where, as in Eitner's acid chrome bath (p. 259), the 
whole of the chromic acid is liberated, the bath may be exhausted 
by a second pack of skins. Many tanners, in order to avoid 
complications of remaking a bath, run away their chrome liquors 

^ Goat-skins for glace kid need tliorough puering to produce a smooth 
grain. It has recently been shown that one considerable result of puering is 
to remove the layer of elastic fibres which are specially abundant in the 
grain of goat-skin, and prevent the flattening out of the grain-surface which 
is required in glazed goat (see p. 223). In some experiments made by Mr 
Wilson, twenty-four hours in a pancreatic bate were necessary for their 
complete solution, but a shorter time would probably be practically 
sufficient. It has been usual in the States to puer as much as sixteen 
hours with dog-dung. 



262 PRINCIPLES OF LEATHER MANUFACTURE 

after once using, and containing all the excess of dichromate which 
has been used. With proper chemical control this is not 
necessary, and is objectionable, not only from its wastefulness, 
but on account of the very poisonous character of the unreduced 
bichromate. Even weak dichromate solutions, especially if 
warm, are liable to cause painful and obstinate eruptions on the 
hands, but this rarely occurs to tanners, as the poisonous action 
of the solution is removed or much lessened on reduction. It is 
well, therefore, to arrange that men who handle skins in the 
chrome bath should subsequently also work in the reducing bath, 
and to avoid, as far as possible, contact of chrome liquors with 
the skin. Methods of analysis of used chrome liquors are given 
in L.I.L.B., pp. 142 et seq. ; L.C.P.B., p. 120. Those for the 
determination of acidity are not however easily applicable in the 
presence of alum and salts of chromic oxide. 

The skins on coming from the chroming bath may be allowed 
to lie for some time without serious injury, but should be carefully 
protected from the action of light, which reduces the chrome 
at the expense of the skin, and renders the subsequent tannage 
irregular. It is found that skins, if brought into a weak or neutral 
reducing bath, are apt to " bleed " or lose chromic acid, which is 
reduced wastefully in the bath. On the other hand a strong 
" hypo " bath is apt to draw the grain and contract the skins, 
owing to the tannage taking place too suddenly. A somewhat 
strong " hypo " bath is therefore often employed as a preparatory 
" dip," the skins being simply drawn through it to fix the chrome 
on the surface, piled on a " horse," and subsequently reduced in 
a bath of ordinary strength. The tendency to bleed is lessened, 
but at the expense of the pelt, by the reduction which takes 
place if the skins are allowed to lie overnight in the chromed 
state. Eitner states that skins chromed in an acid bath {i.e. 
where the whole of the chromic acid is in a free state) show little 
tendency to bleed, and it is probable that the bleeding is mainly 
of undecomposed dichromate. After reduction the skins are 
well washed with warm water, and their later treatment is the 
same as that of skins tanned by the one-bath process, which 
is subsequently described (see p. 264). 

Naturally in practical work the reduction cannot be made to 
proceed rigidly in the definite steps described by Eitner on p. 259, 
but all go on in different proportions together, though by supply- 
ing the acid in proper quantities and at proper intervals, they 
may be made in the main to follow in the given order. Both 
on this account, and because neither the exact amount of chromic 
acid in the skins, nor the sulphurous acid lost by escape into the 



CHROME AND IRON TANNAGES 263 

air can be determined, the reduction cannot be conducted on 
theoretical principles, but the best conditions must be empirically 
determined. Eitner states that 12 parts of thiosulphate dissolved 
in 400 parts of water and 6 parts of (40 per cent.) hydrochloric 
acid are sufficient for 4 parts of bichromate per 100 of wet pelt 
employed in the chrome bath, of which not more than one-half 
to two-thirds is absorbed ; and that if equal parts of bichromate 
and acid are employed in chroming (4 parts of bichromate to 10 
parts of commercial strong acid), the acid used in reducing may be 
lessened to 5 parts. In this case it must not be forgotten that if 
the partially exhausted chrome bath is used for a second pack of 
skins, which are afterwards finished in a bath of full strength, 
nearly the whole quantity of bichromate used in making up one 
bath will be absorbed by the skins. The amount of acid con- 
sumed in reduction will be greater the more rapidly it is added, 
owing to increased escape of sulphurous acid. It is better to add 
the acid, previously diluted with water, in eight or ten successive 
portions, more rapidly at first and more slowly during the latter 
half of the operation, each portion of acid being added as soon 
as no further change of colour appears to be caused by that 
already given. These changes are the more rapid the lighter the 
goods. The colour darkens at first to olive-brown, then gradually 
"becomes green, and finally blue, and when this colour is uniform 
throughout the thickness of the goods, no further acid need be 
added. For goods which have been chromed in an acid bath, 
Eitner states that no acid will be needed for the first twenty to 
thirty minutes. It is important to have a sufficient excess of 
thiosulphate in the bath when reduction is complete, in which 
case the goods may be left for some hours or overnight in the 
bath to complete " neutralisation," but Eitner prefers to use a 
fresh bath of i^ parts of thiosulphate in 400 parts of water for 
this purpose, the bath being used, after settling, for making up 
the reduction bath for the next lot of goods, for which i| parts 
less thiosulphate is used. The goods must be kept in motion 
during reduction, either in a drum or a covered paddle. 

Eitner describes a method of working the two-bath process 
which is very economical both in material and labour, and which 
ought to be quite successful on many classes of goods. ^ The 
prepared pelts, delimed or bated, are placed in a drum with a 
pickle of 10 lb. of salt and i lb. of sulphuric acid in 10 gallons of 
water per 100 lb. of wet pelt, and are run one or two hours 
according to thickness. A solution of 3 lb. of bichromate is now 
added to the pickle in the drum, and run two hours more, or 
^ Jettmar, Handbuch der Chromgerbung, 2nd edition, p. 358, 



264 PRINCIPLES OF LEATHER MANUFACTURE 

until the pelt is evenly chromed and yellow throughout. A 
solution of 15 lb. of thiosulphate (" hypo ") in 10 gallons of water 
is now added to the liquor in the drum, and 5 lb. of diluted 
hydrochloric acid is gradually run in in small portions. The 
leather will first become brown from the deposition of chromium 
chromate, but as more acid is added will take the regular clear 
blue-green. The solution left in the drum should not be more 
than tinged with green. The goods should be well washed, but 
" neutralisation " is probably not absolutely necessary unless the 
goods are to be fat -liquored. 

As soon as the Schultz process proved successful many 
attempts were made to evade the patent by the use of other 
reducing agents than the " hypo " and other salts of sulphurous 
acid which it covered, and almost every imaginable reducing agent 
was patented. Among these the use of hydrogen sulphide and 
acidified solutions of alkaline sulphides, and especially of poly- 
sulphides, ^ proved capable of practical use, though less convenient 
than thiosulphate, but were soon acquired by a combination, 
the Patent Tanning Company, together with Schultz's original 
patents. 

Under these circumstances Martin Dennis, either by fresh dis- 
covery or otherwise, revived the original process of Knapp (p. 256), 
which he patented ^ almost in Knapp's words, and offered a basic 
chrome tanning liquor for sale, without further restrictions on 
its use. This liquor was made by dissolving precipitated and 
washed chromic hydrate (easily prepared by precipitating chrome- 
alum solution with excess of alkali) in hydrochloric acid to 
saturation, and adding washing soda until the solution was 
rendered sufficiently basic. Such a solution may be used on 
skins prepared in the ordinary way by diluting with water and 
strengthened as the tannage proceeds, like a vegetable tan-liquor. 
It is doubtful if the patent was a valid one, as it was known that 
the use of such a solution was not new, and it was only granted 
in America on the representation, which has since been found to 
be mistaken, that chlorides alone were applicable for tanning, 
while Knapp had not restricted his statement to these salts. In 
reality chlorides and sulphates seem equally suitable, but to 
produce similar results the former must be made more basic than 
the latter. In any case the patent could not cover the general 
principle of basic tanning, but only the particular liquor and 

^ " Liver of sulphur " or solutions, made by boiling sodium sulphide or 
soda with excess of sulphur. 

2 Martin Dennis, U.S.A. Pat. 495028, 1893 ; and 511411, 1893, 7732, 
1893. Eng. Pat. Gallagher. 



CHROME AND IRON TANNAGES 265 

mode of preparation specified. It was soon afterwards shown 
by the writer ^ that a good chrome tanning hquor might be 
prepared by direct reduction of dichromate with sugar or other 
carbohydrates in presence of such a limited quantity of hydro- 
chloric acid as to produce a basic salt. Suitable proportions are 
5 mol. HCl to I mol. potassium dichromate, which produces a 
salt approximately Cr2Cl3(OH)3. The solution is easily made by 
dissolving 3 parts of dichromate of potash or soda in a con- 
venient quantity of water, adding 6 parts by weight of con- 
centrated hydrochloric acid, and then cane-sugar or glucose 
gradually till a green solution is obtained, when the whole 
may be made up to 100 parts, and will be approximately of 
the same strength as a 10 per cent, solution of chrome-alum. A 
little heat may be needed to start the reaction, but too much 
should be avoided, as considerable heat is evolved by the oxida- 
tion ; and as much carbonic anhydride is produced, which causes 
the solution to effervesce briskly, the vessel used should be of 
ample size. In place of cane-sugar a good quality of glucose 
may be used, but some samples contain some impurity which 
produces a violet solution which wiU not tan satisfactorily, 
though it may be made to do so by sufficient addition of alkali. 
This liquor is in regular use in many tanneries, producing a good 
quality of chrome calf, but is somewhat variable in its effects 
according to the temperature employed in its preparation, and it 
appears to have no real advantage over a simple solution of 
chrome-alum rendered basic by soda. A somewhat similar pre- 
paration is Eberle's " chromalin," ^ in which some organic 
substance, probably crude glycerine, is used to reduce the bi- 
chromate. The organic matters, and especially the organic acids 
which result from the oxidation of the sugar or glycerine, are not 
without influence on the tanning properties of the liquor. Of 
course these solutions may be rendered still more basic by the 
addition of sodium carbonate. A good stock-liquor, of approxi- 
mately the same strength as that above described, is made by 
dissolving 10 parts of chrome-alum in 80 parts of tepid, but not 
hot, water,^ and adding with constant stirring a solution of 2| to 

^ Leather Trades Review, Jan. 12, 1897. 

^ Compare Eberle's German Patents 1 19042, 1898, and 130678, 1899. 
The last of these appears to be anticipated, at least as regards the use of 
glucose, sugar, and starch, by the writer's publication in 1897, above cited. 

' Later investigations have shown that the temperature of the water is 
unimportant if alkali be added, but chrome-alum dissociates to- some extent 
in hot water, and comparative experiments have shown that solutions of 
the normal salt made with the aid of heat act on skin as if more acid than 
those made in the cold. 



266 PRINCIPLES OF LEATHER MANUFACTURE 

3| parts of washing soda in lo parts of water. The chrome-alum 
dissolves somewhat slowly without the aid of heat, and the 
solution is best made either in a small drum driven by power, 
or by suspending the crystals in a basket near the surface of the 
liquor, so that the saturated solution can descend. 

Eitner ^ has pointed out the important effect that differences 
of basicity have on the tanning properties of. chrome solutions. 
Normal chrome sulphate or chrome-alum colours the leather 
quickly and equally throughout, and swells the pelt on account 
of its practically acid character, but gives a thin and lightly 
tanned leather, from which much of the chrome washes out, 
unless it is at once " neutralised " in alkahne solutions. As the 
chrome solution is made more basic the tannage penetrates 
more slowly, but is heavier and more thorough, the colour is 
darker and bluer, and much less of the chromic salt is removed 
by washing with water. When the basicity becomes excessive 
the solution becomes unstable, and decomposes on dilution with 
water or on contact with the skin into a very basic salt which is 
precipitated, and a more acid solution than that given by a 
moderately basic salt. The effect of such solutions on the leather 
is very unsatisfactory, producing the bad effects both of too acid 
and too basic salts. The pelt is apt to be swollen and lightly 
coloured by the more, acid salt, but at the same time the actual 
tannage proceeds very slowly, and in extreme cases it is difficult 
to tan through, while the surface becomes over-tanned and dis- 
coloured, and the grain often tender and even brittle from the 
incrustation of precipitated basic salt. Eitner likens the effect of 
the more acid liquors to the quickly penetrating and lightly tanning 
vegetable tans, such as gambler, and that of the more basic to the 
heavier tannages, such as valonia ; and within limits, advantage 
may be taken of these facts in adjusting the liquors to the character 
of the leather it is desired to produce. In sulphate liquors, he 
considers the salt CrOH . SO^ as most suited to general use, and in 
the case of chrome-alum this is produced by the use of 286 parts of 
soda-crystals, or 106 parts of dry sodium carbonate (i m.olecule) to 
998 (or practically 1000) parts by weight (i molecule) of chrome- 
alum. (In using washing soda, care must be taken to employ 
clear crystals of the salt, and not those which have become white 
by loss of water, or to allow for the greater strength.) In place of 
soda Eitner makes a similar basic liquor by boiling 1000 parts of 
chrome-alum with 248 parts (i molecule) of sodium hyposulphite 
until the whole of the liberated sulphurous acid is driven off 
and the sulphur deposited. In comparative experiments by the 
^ Gerber, 1901, pp. 3 ef seq. 



CHROME AND IRON TANNAGES 267 

Author no difference could be detected between the tanning 
effects of the two solutions, and that with soda is both cheaper 
and more easily made. If the solution with hyposulphite is not 
boiled a more acid liquor results, in which part of the chromium 
is probably combined with sulphurous acid, forming an unstable 
compound which may prove useful in certain cases. Borax, 
sodium sulphite, and many other weakly alkaline salts may also 
be used. 

Since the war the use of bichromates as oxidants in the dye 
industry has apparently diminished, and, in any case, chrome- 
alum has been not merely dearer, but more dif&cult to obtain ; 
and this has led to a larger production of chrome liquors from 
bichromates, and especially from sodium bichromate. In his 
Cobb lectures to the Society of Arts ^ the writer published a 
method of making a concentrated chrome tanning solution by 
the reduction of a 40 per cent, solution of sodium bichromate 
with gaseous SO2, which he has since learned had been in use as 
a secret by one firm, and which is said to have been first suggested 
by Mr Balderston, the SO2 being made by burning sulphur with 
air under pressure. The following equation represents the final 
result, though there is some doubt as to the actual course of the 
reaction : 

Na2Cr207+3S02 + H20=Na2S04 + 2Cr(OH)S04. 

It will thus be seen that the solution is already basic, and can be 
used on many classes of leather without addition of alkali, but 
contains considerable excess of SOg, which, though it possibly 
does not actually enter into combination with the chromium, 
increases the hydrion concentration, and raises the " precipita- 
tion point " (p. 270) of the liquor. Probably from this cause it 
tans very rapidly, and has come much into use. 

A very similar solution might no doubt be made by the addition 
of Boake's metabisulphite of sodium in small successive portions 
to a solution of sodium bichromate of 4 lb. per gallon. The 
equation for the setting free of SO2 from metabisulphite is 

Na2S205 + HaSO^^NaaSO^ + OH2 + 2SO2, 

so that 1 1 molecules of metabisulphite and of sulphuric acid 
would be required to oxidise one of bichromate, and in round 
numbers 2| lb. of metabisulphite and 2 lb. of concentrated sul- 
phuric acid for 4 lb. of sodium bichromate would be required to 
fulfil the equation, but in practice somewhat more, to provide 
for an excess of SOg in the liquor, and to allow for some escape of 

1 Journ. Roy. Soc. of Arts, 66, 1918, pp. 747, 776. 



268 PRINCIPLES OF LEATHER MANUFACTURE 

the gas. The finished Hquor should smell pretty strongly of 
sulphurous acid, and give a clear blue-green on dilution. 

The effect of weak, and especially of volatile, acids in increasing 
the hydrion concentration of chrome liquors, and so lowering 
the precipitation point, and rendering possible the use of solutions 
extremely basic as regards their fixed acid, deserves more attention 
than it has hitherto received. Burton has shown ^ that even 
carbonic acid has a noticeable effect of this sort. 

Excellent chrome liquors are also made under the patent of 
J. R. Blockey,^ in which the suitably acidifi.ed bichrom.ate is 
reduced by spent tanning material, which is added in excess, 
that which is not oxidised and dissolved being left in the vat for 
use on the next batch, so that no residue remains to be removed. 

Eitner stated that he had made chrome solutions of various 
types, containing organic compounds in combination with the 
chrome salt, which combine with the leather, producing a fuller 
and softer tannage, but he gave no details as to their preparation, 
as they were made commercially by the " Erste Oesterreichische 
Soda-Fabrik " at Hruschau. The writer has found that in some 
cases by the addition of, say, three parts of sugar, or still better of 
glucose, to ten parts of the chrome-alum in making up the basic 
liquor, a much fuller and plumper leather is produced, which dries 
perfectly soft, even without staking or fat -liquoring ; and it is 
probable that many other organic compounds may be found 
which produce similar effects. The addition of very small quanti- 
ties of neutral tartrates or lactates, or of any other hydroxy- 
salts or acids, have, however, a remarkable effect in lowering the 
apparent basicity of the solution and preventing tannage. This is 
due to the property of hydroxy-acids of forming complex ions with 
chromium, which do not tan, and it has been shown by Procter 
and Wilson ^ that solutions of Rochelle salt (sodium potassium 
tartrate) will dissolve all but traces of chrome from chrome 
leather, and leave it in a condition in which it can be boiled for 
glue-making. The RocheUe salt acts best for this purpose in a 
neutral or alkaline condition, and may mostly be recovered by 
suitable acidification, when it is precipitated as the very sparingly 
soluble potassium tartrate, and may again be brought into solu- 
tion by the addition of soda. It is highly probable that the 
unsatisfactory tanning liquors produced by direct reduction with 
some samples of glucose are due to the presence of small quantities 
of some such organic acid produced during the oxidation. It has 

^ Burton, J.S.L.T.C., 4, 1920, 205 ; Burton and Hey, ibid., 272. 
^ Blockey, Eng. Pat. 131772; J.S.C.I., 1919, 783 A. 
^ Procter and Wilson, J.S.C.I., 19 16, p. 156. 



CHROME AND IRON TANNAGES 269 

been found that these solutions may be made to tan by the hberal 
addition of soda. It is probable that more satisfactory results in 
chrome tanning will be attained by the direct addition of known 
organic substances to basic liquors of definite constitution than 
by the somewhat uncertain products of organic oxidations. 

The quantity of salt to be added depends on the qualities 
desired in the leather, and upon whether chloride or sulphate 
liquors are employed ; salt in chloride liquors increasing the soft- 
ness of the leather, but in excess tending to flatness, while in 
sulphate liquors it practically diminishes their basicity by con- 
verting the chromium sulphate into the equivalent chloride, 
which, as Eitner points out, behaves as a less basic salt, and 
hence but little advantage is to be gained from its use. It is 
best to begin with a very weak liquor to avoid drawn grain, 
and for the same purpose a preparatory tannage with alumina 
salts, or an addition of alum or sulphate of alumina and salt, may 
be made to the first liquor, as the attraction of the chrome salt 
for the fibre is sufficient to produce a chrome tannage, even in 
presence of excess of alumina salts. Ten lb. of chrome-alum 
will tan about 100 lb. of wet pelt, but more must be used for the 
first parcel ; to avoid loss of time, the skins may be tanned 
out in a pretty strong liquor. The bath has a tendency to 
become acid by use, and before strengthening it may be necessary 
to add some more soda solution. Little, if any, additional salt is 
required, as it is only absorbed by the skins to a small extent, 
probably as chromic chloride. As the liquors gradually become 
charged with sulphates, it is best to work them out like bark 
liquors, and not to go on strengthening the same liquor in- 
definitely. If old liquors are used for green goods, it is not 
necessary to neutralise them with soda before use, as Eitner has 
shown that less basic liquors colour more evenly and with less 
tendency to produce drawn grain. 

The cause of this increase of acidity in the liquors is that the 
basic chrome salts hydrolyse on dilution, and that the acid 
diffuses much more rapidly than the basic chrome, so that at 
first the liquor must be kept acid to prevent the precipitation 
of the basic salt ; but as the process proceeds the hide becomes 
saturated with acid, which is not further absorbed, while the 
fixation of the chrome salt still continues, and the excess of acid 
tends to prevent a full tannage. It is therefore necessary to 
maintain the basicity of the final liquors, either by strengthening 
with very basic solutions or by the addition of alkalies. This 
is controlled by determining the chrome by oxidation {L.I.L.B. 
and L.C.P.B., p. 122) and titration with potassium iodide, and 



270 PRINCIPLES OF LEATHER MANUFACTURE 

subsequently titrating the acid with caustic soda in a boiling 
solution, and calculating the " basicity number." This number 
is the proportion of sulphuric acid to the atom (52) of Cr. For 
the normal salt it is therefore 144, and diminishes as the basicity 
increases, the practical values ranging about 96, corresponding to 
S04Cr(0H). It was suggested by Stiasny when working on 
chrome sulphates, and of course was quite appropriate for his 
purpose, but is less convenient when working with chlorides or 
other salts. A much better way would be simply to give the 
proportion of (OH) to Cra (104). In this case the value of chrome 
hydrate Cr2(0H)g would be 6, with less values for less basic 
solutions. It is preferable to adopt Crg rather than Cr, as it 
gives a larger scale of whole numbers, and fits better with sul- 
phates, while chrome chloride is very frequently written CraClg. ^ 
A practical way for the tanner of determining the basicity of 
liquors is that of McCandlish's " precipitation point," which has 
a more direct relation to the tanning properties of a liquor than 
the exact acid determination, which varies in its effect with 
different acids. The chrome liquor is filtered perfectly clear with 
the addition of a little kaolin, and 10 c.c. is titrated with constant 
stirring with a N/io or N/20 solution of sodium carbonate, or of 
whatever alkali {e.g. borax) is used for neutralisation, until a 
permanent turbidity is produced which does not disappear on 
stirring or shaking. This gives the largest quantity of alkali 
which could be added. How much the tanner should stop short 
of this in practice is a question of experience, but a higher margin 
must be left with green goods than with hides nearly tanned. In 
the early stages the absorption of acid by the skin is frequently 
so large that a portion of the basic chrome salt is precipitated and 
the liquor becomes turbid. 

Chrome sole leather has become of some importance, and when 
well manufactured it is probably the most durable and water- 
proof leather known for the purpose, though it is to be feared 
that all supplied during the war did not merit this character. It 
is usually tanned by suspension in the ordinary basic chrome 
liquors, and should at least be well washed and neutralised 
slightly, perhaps with thiosulphate, dried to a sammed condition 
and rolled or struck out, and then thoroughly dried (often nailed 
on boards for the sake of flatness) at a rather high temperature, 

1 It has been proposed in America to express the " acidity " of chrome 
Uquors in an analogous but inverse way. CrgClg and Cr2 (804)3 have an 
" acidity "=6, Cr(OH)Cl2 and Cr(OH)S04=4, Cr(0H)2Cl and Cr2(OH)4S04 
=2, whilst the acidity of Cr2(OH)6 is zero. The number here is that of 
the monovalent acid radicals combined with aCr. 



CHROME AND IRON TANNAGES 271 

and impregnated by suspension in a bath of melted waxes, and 
hung over the hot tank to drain thoroughly, when, after cooling, 
it is ready for use. 

A method of tannage which gave excellent results on a small 
scale was as follows : — The butts, after rounding, but without 
deliming, were placed in a 10 per cent, solution of alumina 
sulphate till well penetrated and slightly swollen, and were then 
treated with a pretty strong basic liquor, made by reduction of 
sodium bichromate with sulphurous acid, in which they tanned 
very rapidly. The leather was of a light and even colour, and 
decidedly thicker than test-pieces tanned in basic liquors in the 
usual way after deliming. Both liquors could be used repeatedly. 

The best waxing mixture is somewhat a secret, and mixtures 
with too much paraffin wax slip badly in wet weather. The 
writer has had good results from a mixture of two parts of pale 
rosin and one of paraffin wax, with a little tallow to soften it 
sHghtly. 

Basic chrome liquors, such as have been described, may also 
be used in chrome combination tannage. It is generally best to 
let the light vegetable tannage precede the chrome, and lightly 
tanned skins, such as " Persians " and East India kips, acquire 
many of the qualities of chrome tanned leather by the treatment. 
The effect is stili further increased by a previous detannisation 
of the leather with alkaline solutions (see p. 379). Many firms 
now supply basic chrome liquors ready prepared for use. 

The time required for chrome tannage will of course vary with 
the thickness of the goods, and for calf-skins will usually extend 
over some days, though it can be much quickened by drumming. 
The tannage is generally best accomplished in the paddle, but can 
be carried out by frequent handling in pits or tubs, or, where very 
smooth grain is important, by suspension. When the goods come 
out of the final liquor they may be allowed to lie in pile for 
twenty-four hours, or even for some days, with advantage, as the 
surplus chrome liquor is pressed out, and the tannage becomes 
more complete. They are then washed with plenty of warm 
water, till it ceases to be coloured with chrome. They may be 
kept for an almost unlimited time in a wet condition, as they do' 
not bleed, and have little tendency to heat even in pile. They 
have now reached the stage at which we left the " two-bath " 
leather, and the subsequent treatment may be the same in both, 
cases. 

Although by both processes the chrome salt fixed in the fibre 
is of a decidedly basic character,, it still contains enough acid to 
act injuriously on the leather in cour^jQ of ti=m,e, and to lead to 



272 PRINCIPLES OF LEATHER MANUFACTURE 

serious inconveniences in its subsequent treatment. Before pro- 
ceeding further this excess of acid must be removed or neutral- 
ised, and it is not too much to say that most of the troubles 
experienced in fat-liquoring arise from neglect or mistake in 
the washing and neutralisation. The difficulty in the process 
arises from the fact that while the acid should be reduced to a 
mere trace it must not be entirely removed/ as chromic oxide 
itself does not seem capable of tanning, and at any rate the 
effect of excess of strong alkalies is at once to render the leather 
hard and pelty. Borax is one of the safest neutralising materials, 
about 3 per cent, on the wet weight of the pelt being required 
in not more than | per cent, solution. Eitner recommends the 
use of silicate of soda, which, sold as a solution of sp. gr. 1-5, is 
somewhat stronger and much cheaper than borax. Hyposulphite 
of soda and whitening together neutralise more rapidly and com- 
pletely than either alone. Other salts of weak acids may also 
be used, the acids exercising a regulating influence which pre- 
vents neutralisation going too far. Sodium carbonate or bi- 
carbonate, or ammonia, may also be used, but with these it is 
difficult to get even " neutralisation," or to avoid the risk of 
carrying the process too far. Stiasny's mixture ^ of 2 per cent, 
of soda crystals and 2 per cent, of ammonium chloride or sul- 
phate, to which 2 per cent, more soda and i per cent, more 
ammonium salt may be added if the neutralisation is not sufficient, 
is perfectly safe and efficient, and cannot over-neutralise, and 
statements about its expense are unfounded, since the mixture 
can be used repeatedly, merely strengthening with soda and a 
little ammonium salt as required, since it is really the soda which 
neutralises, and the ammonia acts only as a " buffer " to regulate 
the hydroxyl concentration. Even a thorough drumming with 
a milk of " whitening " (calcium carbonate) or magnesia is 
effective. With the latter there is no danger of overdoing the 
process, but in some cases the adhering whitening and preci- 
pitated calcium sulphate are troublesome in later operations. In 
any case the neutralising should only be carried so far that the 
skins show no acid reaction to litmus paper ; and whatever means 
of neutralising are employed, the fat -liquoring should take place 
without delay, before the more acid liquor in the centre of the 
skin has time to diffuse again to the surface. 

It is probable that one of the great causes of difference between 
" one-bath " and " two-bath " leathers is the presence of free 
sulphur in the latter. This may also be introduced into " one- 

1 Procter and Griffith, Journ. Soc. Chem. Ind., 1900, p. 223. 

2 Colleg., II, 1912, p. 293. 



CHROME AND IRON TANNAGES 273 

bath " leather, by treating it in the wet chromed state, without 
washing out the chrome hquor, with excess of a solution of 
hyposulphite, or of an alkaline polysulphide, which at the same 
time will neutralise the skin. The more acid the chrome liquor, 
the greater the quantity of sulphur which will be introduced. 
The simplest means of distinguishing " two-bath " from " one- • 
bath " tannages is to test for the presence of sulphur, by wrapping 
up a silver coin with a piece of the leather in paper, and leaving 
the parcel for an hour in the water-oven, or some other warm 
place, when the presence of sulphur will be shown by the blacken- 
ing of the coin. Of course a sulphurised " one-bath " leather 
will give the same reaction. 

The leather must now be dyed and fat-liquored. Which of 
these two operations should be first undertaken will depend on 
circumstances. Most leathers dye more easily before fat -liquor- 
ing, but if acid dyes which are soluble in the alkaline fat-liquor 
are used, a good deal of colour is often lost. This may be com- 
pensated by dissolving a suitable aniline (acid) colour in the fat- 
liquor. " Bluebacking " is generally done before fat -liquoring by 
drumming with methyl violet or some other aniline colour (with 
or without logwood, which gives alone a very dark violet) . Any 
shaving or splitting required must of course be done before 
bluebacking. 

The fat -liquor is an emulsion of soap and oil, which for chrome 
leather should be as neutral as possible if the neutralising has 
been thorough ; but if any acid be left on the skins, a neutral 
fat -liquor will be precipitated as a greasy mass. This can some- 
times be remedied by the addition of a little ammonia or borax, 
or by re-fat -Hquoring with soap solution only, but if the washing 
of the skins has been incomplete, and soluble chrome salts 
remain, the mischief is almost irretrievable, as sticky chrome- 
soaps are formed, often coloured with the aniline violet, which 
adhere to the skins, and which can scarcely be removed by any 
solvent which does not injure the leather. As regards the soaps 
and oils used, there is considerable latitude : i| per cent, of 
castor-oil soap and f per cent, of castor or olive oil on the wet 
weight of the pelt has done good service in my hands, but many 
manufacturers em.ploy soft soaps, curd soaps, etc., with castor, 
olive, cod or neatsfoot oil, and sometimes sod-oil or degras. 
Eitner considers ohve oil and ohve oil potash soap the most 
suitable, and particularly warns against the use either of drying 
oils or of oils containing tallow (such as neatsfoot), which are 
not only apt to cause a white efflorescence, but to give the leather 
a disagreeable rancid smell. Fish oils are unsuitable, but mineral 

18 



274 PRINCIPLES OF LEATHER MANUFACTURE 

oils are often useful constituents of fat-liquors. Wool-fat also 
makes a good fat-liquor, but is unsuitable for goods which are 
to be glazed. " Turkey-red oil " (which is sulphated castor) may 
be used as a fat -liquor, simply mixed with warm water without 
-soap, and has been recommended where delicate colours are to 
be dyed after fat -liquoring ; but it is said to have an unsatis- 
factory after-effect, hardening and tendering the leather.^ Some 
soaps made from the saponifiable part of wool-grease, such as 
" Lanosoap," also act well in conjunction with olive, castor, or 
other oils. Where leather is to be glazed, the amount of fat- 
liquoring must be kept very moderate. Fat-liquors should be 
thoroughly emulsified, and are generally used warm. They 
penetrate better if the leather is partially dried by sleeking out, 
or pressing, or cautious " samming," but the leather must not 
be completely dried out before fat -liquoring and dyeing, unless it 
has been previously treated with glycerine, glucose, treacle, or 
some deliquescent salt, which will enable it to be wet back. 
Chrome leathers are not " waterproof," as has often been stated, 
unless rendered so by treatment with soaps and greases, and are 
apparently easily wetted, but the fibre will no longer absorb water 
after thorough drying, and consequently will neither dye nor stuff 
satisfactorily. In order to enable chrome leather to be kept in 
an undyed condition glycerine or syrup is sometimes mixed with 
the fat -liquor, but as the watery portion of this is not generally 
completely absorbed, the process is somewhat wasteful. Mr M. C. 
Lamb avoids this difficulty by applying a solution of glycerine 
to the grain side with a sponge after fat -liquoring. In this case 
the leather may be dried sufficiently for staking or shaving 
without risk. 

Chrome leather can be dyed by many of the acid aniline colours 
without a mordant. Basic colours are only fixed when the 
leather has been first prepared with a vegetable tannin, gambler 
or a mixture of gambler and sumach being the most suitable. 
Considerable care must be employed in the application of tannins 
to chrome leather, as they have a tendency to harden it and 
diminish its stretch, or even to render it tender, but traces of 
tannin in the dye probably facilitate glazing. Before dyeing 

1 This is somewhat doubtful if the oils are satisfactorily prepared and 
free from excess of acid, and in any case sulphonated oils, not merely 
castor but other vegetable oils, have come largely into use, and as such 
fat-liquors are not to any great extent alkaline, and may even be acid, 
they do not strip dyes, and are not so readily precipitated if the neutralisa- 
tion is not quite complete. Sulphonated fish-oils are largely used in other 
branches of the trade and have great emulsifying powers, but the writer 
has no experience of their use on chrome leather. 



CHROME AND IRON TANNAGES 275 

it is advantageous to fix the tannin with tartar emetic, or, for 
browns and yellows, with titanium potassium oxalate solution, 
which itself gives a good yellow-brown with tannin. In place of 
employing the tannin and titanium salt in two separate baths 
they may be combined, using a weight of the gambler or tanning 
extract (oakwood, chestnut, etc.) about equal to that of the 
titanium salt, or titanium tanno-oxalate solution may be used. 
Chrome leather may be dyed with the various dye-woods, which 
are mordanted by the chromium present, but the colours are 
mostly dull, that of logwood being nearly black. A good black 
of a very permanent character is obtained by dyeing with log- 
wood, and saddening with a hot solution of titanium oxalate in 
the drum. Iron-logwood blacks are much used on chrome 
leather, often in conjunction with coal-tar dyes, and a little iron- 
alum added to the chrome liquor in tanning will facilitate dyeing 
the skins black with logwood and help it to penetrate through 
the leather, which is sometimes desired. Several aniline blacks 
give very satisfactory blacks by brushing or dyeing. For suit- 
able dyes consult App. C. 

Chrome skins may be glazed in the ordinary way with blood 
or albumen mixtures under glass or agate, but require good 
pressure and repeated seasonings and glazings, and much care is 
required in fat -liquoring. The glazing is often assisted by the 
previous application of barberry juice {epine vinette) or of lactic 
or tartaric acid solution with a trace of sugar. Much of the 
difficulty which has been experienced in glazing chrome leathers 
is due either to the natural fat of the skin, or to oils used in fat- 
liquoring in excessive quantity or of unsuitable character. 

Iron tannages may be very shortly dismissed, as their practical 
interest is at present mostly either historical or prospective ; and 
the salts of iron have been discussed in connection with those of 
chrome. It is well, however, here to insist again on their action 
as oxygen carriers, which is often very destructive, especially on 
vegetable tanned leather. Ferric salts in contact with more 
easily oxidisable matters readily give up oxygen, and pass into 
the ferrous state ; and especially does this happen in the presence 
of organic matter under the influence of sunlight. Thus iron- 
salts often act as carriers of oxygen, and oxidisers of organic 
matter, absorbing oxygen from the air, and giving it up again 
under the influence of light or heat to any organic matter with 
which they are in contact. Thus when used in dyeing black, it 
is important that there should be considerable excess of tannin 
or of colouring matter present. There is also little doubt that 
the presence of ferric salts in leather blacks has a great tendency 



276 PRINCIPLES OF LEATHER MANUFACTURE 

to cause the resinification of the oil, known as " spueing," by 
promoting its oxidation. 

The ferric salts are characterised by giving blue-black or green- 
black compounds with tannins and with many other allied 
bodies, while the corresponding ferrous compounds are mostly 
colourless, though they rapidly oxidise and darken on exposure. 

Ferric iron, like alumina, forms an " alum," a double sulphate 
of iron and potassium, Fe2(S04)3K2S04,24Aq, forming fine pale 
violet crystals, but dissolving to a yellow-brown solution. (It 
must be distinctly understood that iron-alum and chrome-alum 
contain no alumina, but are simply called alums because of their 
similarity of constitution, iron or chrome taking the place of the 
aluminium. Iron-alum, in conjunction with salt, can be used 
for tanning, giving a pale buff-coloured leather very similar to 
an ordinary alum leather. Thus the presence of a small quantity 
of iron in an alum used for tawing is of no consequence, except 
as affecting the colour of the leather. In impure sulphate of 
alumina, such as " alumino-ferric," it, however, generally exists 
in the green ferrous state, and only acquires tanning properties 
on oxidation. Without common salt iron-salts are still less 
satisfactory tanning agents than those of alumina under the same 
conditions, as the acid is yet more loosely held, and though basic 
ferric salts are taken up in considerable quantities by hide, the 
leather produced is thin, and usually brittle. Professor Knapp 
devoted much study to the production of a commercial sole 
leather by basic iron-salts, and took several patents, which did 
not prove practically successful, though the brittleness was to 
some extent overcome by the incorporation of compounds of iron 
with organic materials such as blood and urine, of iron-soaps, 
and of rosin and paraffin in the leather. Like most mineral 
tannages, the process was far more rapid than that with vegetable 
materials. Knapp's basic tanning liquor was made by the 
oxidation of ferrous sulphate with a small quantity of nitric acid. 
Patents have been taken for the oxidation of ferrous sulphate 
by peroxide of manganese in presence of sulphuric acid, which 
produces basic ferric sulphate in mixture with manganese sul- 
phate, which has also some tanning properties. Attempts have 
also been made to tan by treatment of the hide with solutions of 
ferrous sulphate and subsequent exposure to the air, in order to 
oxidise the iron on the fibre and convert it into a basic ferric salt, 
but have not proved of any commercial value. 

Owing to the scarcity of tanning materials in Germany during 
the war and the great demand for leather, considerable attention 
was paid to iron-tanning, and quite a number of patents were 



CHROME AND IRON TANNAGES 277 

taken out, and some of them claim to be successful in producing 
merchantable leather. Dr J. Bystron ^ treats first with ferrous 
sulphate, and oxidises on the fibre with ferric nitrate. The 
" Chemischen Fabriken," Worms, patent the use of ferric for- 
mates ; 2 Moos and Kutsis ^ of Stuttgart, the use of iron-salts in 
conjunction with sulphite-cellulose liquors ; and Dr Otto Rohm 
has taken a variety of patents * for iron tannages in conjunction 
with aldehydes and other things. The great difficulty appears 
to be to produce a leather which will not become brittle on 
keeping. More hopeful are the numerous patents which have 
been taken for combinations of chrome and iron tannages, and 
there is no doubt that a merchantable leather can be made on 
these lines. For details and a good general review of iron 
tanning processes the reader may be referred to Ing. Josef 
Jettmar's little book Die Eisengerbung, published by Schulze 
and Co., Leipzig, 1920. No attention appears to have been 
paid to combined iron and aluminium tannage, which would 
seem worth experiment, as the combined salts would be very 
cheap, most natural clays containing both metals, and their 
properties being in some ways complementary. It is said that 
iron tannages cannot be neutralised with alkalies without 
destruction. 

1 D.R.P. 255324, 1910. 2 Aust. Pat. 80067, 1918. 

^ Swiss Pat. 81781, 1918. 

* Swiss Pat. 74849, 1916; 75618, 1916 ; Aust. Pat. 77867, 1919. 



CHAPTER XVIII 

VEGETABLE TANNING MATERIALS 

As has been stated in the previous chapter, our knowledge of 
the chemistry of tannins is not sufficiently advanced to render 
possible any strictly chemical classification, while an additional 
complication arises from the fact that very different tannins may 
coexist in the wood, bark, fruit, galls, etc., of the same plant. It 
therefore seems best to follow the example o£ Professor Bernardin 
in his Classification de 350 matieres tannantes} and arrange the 
plants under the orders of the natural system of botany, as has 
already been done by von Hohnel ^ and A. de Lof,^ and con- 
siderable use has been made, not only of these books, but of the 
recent exhaustive work of Dekker ^ on the vegetable tanning 
materials, and of the various lists which have from time to time 
appeared in the scientific press. In the following pages only 
those materials which from their high percentage of tannin, or 
from some other cause, seem of commercial interest or value are 
included, as the tannins are so widely distributed in the vegetable 
kingdom that any exhaustive list would be quite out of the 
question. 

Tannins are not confined to any particular part of the plant, 
though they are usually most abundant in barks and fruits. 
Insect-galls are often very rich in tannin, usually gallotannic 
acid ; while in several cases woods are of commercial importance 
from their cheapness, though the percentage of tannin they con- 
tain is not generally high. The function of tannins in the 
vegetable economy is not well understood. In some cases they 
are probably a waste product of plant-life, and may help to 
ward off attacks of insects. They usually exist as cell-contents, 
and as vegetable cells have frequently thick and impermeable 
walls, and the diffusive power of tannins is low, much time is 
required for extraction, unless the cells have been previously 
crushed or broken. 

1 Gand, 1880. ^ Die Gerberinden, Berlin, 1880. 

^ Matieres tannantes, Halle aux Cuirs, Paris, iSgio. See also Agricultura 
Ledger, 1902, No. i (Government Printing Office, Calcutta, 6d.), by Mr. 
D. Hooper, which contains much valuable information, and various Indian 
Forestry Reports. * Dekker, Die Gerbstoffe, Berlin, 191 3. 

278 



VEGETABLE TANNING MATERIALS 279 

It would be beyond the scope of this text-book to describe 
in detail the structures of the tannin-yielding parts of plants ; 
but barks are of such general importance, that some particulars 
seem desirable. 

The detailed structure of bark varies greatly in different trees, 
though its general principles remain unaltered. One of the best 
short accounts of these is given by Professor H. Marshall Ward 
on page 199 of his little book on Timber and Some of its Diseases,^ 
and further information may be found in Van Tieghem's Traite 
de Botanique and other works on structural botany. 

With regard to detailed structure of various tanning barks. 
Die Gerherinden of von Hohnel ^ is one of the best authorities. 

The inner surface of the bark of a young tree, or twig, consists 
of a layer of soft and living cells resting on the outer surface of 
the wood, called the cambium. These cells multiply by division 
[cp. p. 11), and produce from their inner surface the successive 
annual layers of wood, and on their outer a fibrous tissue called 
the bast {phloem), consisting of lengthened cells, and tubes with 
perforated divisions (sieve-tubes) which convey sap, and mostly 
run in the direction of the branch, but are crossed transversely 
by cells in a line with the medullary rays of the wood. All these 
cells when first produced in the cambium-layer have thin and 
soft cellulose walls, but the inner layer forming the wood becomes 
lignified, or hardened, by deposits of lignine on the interior of the 
cell-walls, while their contents of living protoplasm disappear. 
The outer layer forming bast remains much softer and more 
fibrous, and retains its vitahty longer. The outer surface of the 
young branch is covered by a thin layer of flat cork-like cells 
forming the epidermis, developed from the growing tissue of the 
bud, beneath which is a layer of growing cells frequently called 
the cork-cambium. This produces, on its inner side, a layer of 
soft, juicy, thin-walled cells (parenchym), which are living and 
capable of growth, and contain protoplasm and often chlorophyll, 
to which the green colour of young twigs is due. This layer at 
first rests on the bast. On the outer side, the cork-cambium 
produces corky layers beneath the epidermis. The section of an 
oak-twig is shown in fig. 47. 

As the tree grows, it is obvious that the corky epidermis which 
grows in thickness, but not in breadth, must become distended 
and finally ruptured. In some cases the surface is renewed by 
fresh corky layers constantly developed below it, and then the 
bark remains smooth and unfurrowed, as in the beech and young 
oak, or in the birch, from which thin corky layers are continually 
1 Macmillan & Co. • ^ Die Gerherinden, Berlin, 1880. 



28o PRINCIPLES OF LEATHER MANUFACTURE 

peeling ; or it may produce a thick layer of cork, as in the cork- 
oak. In many cases, and especially in older trees, the outer or 
primary layer of cork-cambium ultimately dies for want of 
nourishment, and a fresh cork-producing layer is developed in 
the still hving parenchym. As cork is practically air- and 
water-proof, the new layer cuts off from its source of nourish- 
ment and kills all the parenchym exterior to it. In some cases 
this peels off, as in the Oriental plane [Platanus), but usually it 




Fig. 47. — Section of Oak Twig, drawn by Professor Bastin ; C, corky 
layer; t, tannin-cells ; /and St, stone-cells, etc. ; Ca, cambium; Mr, med- 
ullary ray; P, pith. 

forms a constantly increasing coat of dead tissue forming the 
" ross " or "crap" (Ger. Borke), which, as it cannot increase in 
breadth, becom.es deeply fissured as the tree becomes old. In 
some cases the new growing layer or secondary cork-cambium 
forms a complete coating parallel with the first, but more often 
it consists of a series of arcs convex towards the tree and cutting 
the primary cork-cambium at various places, so as to divide the 
tissue outside itself into scales. Later on the process repeats 
itself, new arcs forming inside the first, and cutting off further 
portions of the parenchym.^ In this way the cork-forming layer 
gradually sinks deeper and deeper into the bark, till it frequently 

1 This structure is particularly obvious in the outer bark of the hemlock 
and Aleppo pines. 



VEGETABLE TANNING MATERIALS 281 

passes even into the bast -layer, and very complicated arrange- 
ments of tissue result, in which corky layers from the secondary 
cork-cambium are interspersed with bast-cells and sieve- 
tubes. 

As a rule the outer and dead part of the bark contains but 
little tannin, though to this there are exceptions, as, for instance, 
in the hemlock and Aleppo pines. It always contains a large 
proportion of dark colouring matters (reds, phlobaphenes, p. 342). 

Cork consists of thin, and often roughly cubical, cells, which 
are filled with air, while tannin is usually contained in somewhat 
similar cells with thicker walls. The walls of many vegetable 
cells are perforated with fine holes, and become thickened by 
internal deposits of hard ligneous matter which sometimes almost 
fill the entire cell (" stone-cells "). Bark-cells often contain 
starch-granules, frequently of peculiar and characteristic forms 
(which are easily recognised by the blue colour produced on 
treating the preparation under the microscope with a drop of a 
solution of iodine in potassium iodide), as well as crystals of 
oxalate of lime and other matters. These, and the form and 
arrangement of the cells as seen in sections under the microscope, 
form useful marks of recognition of the various barks. Tannin 
is most easily detected by staining, before cutting sections, with 
a solution of ferric chloride in absolute alcohol. 

Apart from microscopic characteristics, the external appear- 
ance of barks, both to the naked eye and by the aid of a lens, 
forms a valuable means of recognition. The arrangement of the 
bast and corky layers, the remains of epidermis, or the form 
and character of the fissures, and of the lenticels or small corky 
protuberances which take the place of stomata in the epidermis, 
should be observed. 

Space does not permit of any detailed account of the structure 
of fruits, woods, and leaves, which are also cellular structures in 
many respects resembling the bark. The cuticles of leaves, and 
especially the stomata or breathing pores, and the hairs are 
often very characteristic {cp. figs. 59-66, and p. 309). 

Valuable hints may also be obtained from the chemical reactions 
which are described on pp. 70 et seq. L.I.L.B. 



282 PRINCIPLES OF LEATHER MANUFACTURE 



Botanical List of Tanning Materials ^ 

CONIFERS, Pines, Cypresses, mostly containing Catechol- 
tans, yielding reds 

Abies excelsa, Lam. {Pinus Abies, Pinus Picea, Picea vulgaris, 
Link.), Norway Spruce. Fr. Faux sapin ; Ger. Fichte, Rottanne. 
The source of the so-called larch-extract, and a principal tanning 
material of Austria. Contains 7 to 13 per cent, of a catechol- 
tannin and much fermentable sugar, and on this account is use- 
ful for swelling and colouring, but does not tan heavily. English 
and Scandinavian bark does not seem much utilised. Best 
bark 2 to 8 mm. thick ; smooth, yellow inside, with reddish- 
brown ross outside. For detailed description of structure see 
von Hohnel, Die Gerberinden, p. 35. 

Abies pectinata. Silver Fir. Fr. Sapin ; Ger. Edeltanne, 
Silbertanne, Weisstanne. Used to a limited extent, but apt to 
be confused with spruce. Contains 6 to 15 per cent, iron-blueing 
tannin. Used in Styria, Austria, Russia. Without " ross," but 
silver-grey and smooth outside. (Von Hohnel, Die Gerberinden, 
p. 40 ; " Gerber," 1875, p. 375.) 

Abies {Pinus, Tsuga) canadensis, Hemlock Fir (fig. 48).^ The 
principal American tanning material, and source of hemlock 
extract ; averages 8 to 10 per cent, of a catechol-tannin, but 
variable ; 18 per cent, reported, possibly from a different species. 
Abundant in Canada and the Northern and North-western States 
of America. The bark of old trees, which is principally used for 
tanning and extract-making, is 2 to 4 cm. thick, smooth and 
yellow within, greyish and deeply fissured without. The ross, 
which is red and thick, contains a considerable quantity of tannin, 
with much dark-red phlobaphen. It does not differ in structure 
from the inner living and yellow " flesh." The bark is easily 
recognisable by its well-marked concave lamellae of cork, cutting 
off successive layers of " ross " of several millimetres in thickness. 
(Von Hohnel, Die Gerberinden, p. 42.) {Cp. p. 280.) 

Abies alba {Picea alba), White Spruce, North America. In 
character of tree and bark very similar to Norway spruce. 

Larix europcea D.C. {Abies or Pinus Larix), Larch. Fr. 

1 The percentage of tannin given where the source of information is not 
stated must in many cases be regarded as uncertain, many analyses having 
been made before the introduction of modern methods, but those quoted 
as having been done in the Author's laboratory are of recent date, and have 
been made by the hide-powder method. 

2 Bastin and Trimble's "American Coniferae," American Journal of 
Pharmacy. 



VEGETABLE TANNING MATERIALS 283 

MeUze ; Ger. Ldrche. Contains 9 to 10 per cent, pale catechol- 
tannin, mild and suitable for light leathers. Used, especially in 
Scotland, for basil tannage. 

Pinus halepensis, Aleppo Pine. An important tanning material 




Fig. 48. — Hemlock Fir (Tsuga canadense). 



of the Mediterranean coasts. The outer bark, stripped like cork 
from the living tree {Scorza or Cortegia rosso), is a deep red 
tannage, and contains about 15 per cent, of tannin very similar 
to hemlock. It is largely used in the island of Syra. The inner 
and fleshy part of the bark, only obtained when the tree is cut, 
is Snouhar or Snohar bark, containing up to 25 per cent, of 
lighter coloured tannin. This bark is reddish-brown, and pretty 
smooth on both sides, except for shell-like depressions on the outer 



284 PRINCIPLES OF LEATHER MANUFACTURE 

surface. The " scorza rossa " is dark red-brown internally, grey 
and irregular outside, frequently very thick, and divided into 
successive layers of i to 2 mm. thickness by cork lamellae 
(p. 280). (Von Hohnel, Die Gerberinden, p. 44.) In appearance 
the tree resembles the Scotch fir. 

Pinus tceda, America ; P. Laricio, Austrian Pine ; P. mari- 
tima, Mediterranean ; P. Cembra, Alps, Tyrol, 3 to 5 per cent. ; 
P. sylvestris, Scotch Fir. Ger. Kiefer ; Fr. Pin sauvage, 4 to 5 per 
cent. P. longifolia Roxb., India, 11 to 14 per cent. 

Juniperus communis, Juniper. Bark used in Russia. 

Podocarpus elongata and Thunbergii, Cape of Good Hope ; 
Geelhout, Yellow woods. 

Phyllocladus trichomanoides , New Zealand ; Tanekahi, Tarse- 
kahi, Kiri-toa-toa, " Golden Tan." Used in dyeing glove-leather. 
Tannin, 28 to 30 per cent. ; gives green-blacks with iron. 

P. asplenifolia, Tasmania, Celery-topped Pine ; 23 per cent. 
Phyllocladus belongs to Yew family. 

P. rhomboidalis, Tasmania, 20-6 per cent, bark (Dekker), and 
red colouring matter. 

Callitris calcarata, Australia ; bark 21 per cent, tannin. 

Most pine barks contain more or less tannin. 



LILIACEiE 

Scilla maritima, Squill. Tannin stated from 2 to 24 per cent. 
More valuable for pharmacy. 



Areca catechu, Betel-nut Palm of India. Yields a species of 
cutch of no importance for tanning. Fruits yield an extract 
" Rossa," and contain 10 to 15 per cent, tannin, but no catechin. 

Sabal serrulata, Saw Palmetto of Florida (Trimble). (" Dwarf " 
palmetto is 5. Adansonia.) Palmetto root has been much talked 
of as a tanning material, and makes a light-coloured leather. 

An extract has been made from the roots of the Saw Palmetto, 
which grows freely in the Southern States of America, and is 
especially abundant on the east coast of Florida. The plant is 
an evergreen, the stem growing flat along the ground, being held 
in place by numerous roots each the size of a pipe-stem. The 
leaves are fan-shaped and ribbed, and 2 to 3 feet in diameter. In 
its hardihood the palmetto resembles a weed, as the leaves may 
be cut off quite close to the stem without damaging the plant, 



VEGETABLE TANNING MATERIALS 285 

which will grow freely on poor sandy land which is worthless 
for other purposes. The average yield is stated to be about 
10 cwt. to the acre, but in good seasons and with rich land over 
a ton per acre has been obtained. 

The air-dried leaves contain about 13 per cent, of tannin, but 
the results obtained by different chemists vary from 5 to 20 per 
cent. Possibly these variations are caused by the different 
amounts of moisture in the various samples. Trimble (1896) 
found very little tannin, and does not consider the plant of 
importance. 

The leaves must be treated with a solution of caustic soda 
to remove the glossy siliceous shield which covers them and 
prevents their being easily extracted. After the tanning matter 
has been extracted the remaining fibre can be profitably disposed 
of to jpaper and rope manufacturers. 

As the supply of palmetto is very large it was expected that it 
would, to a considerable extent, substitute the employment of 
gambler, and in the United States the extract met with a con- 
siderable sale, but does not seem to have been sent to England 
in any quantity. Samples of the extract examined by the 
Author analysed from 16 to 22 per cent, of tanning matter and 
several per cent, of mineral matter, and produced a very soft and 
mellow leather of good colour. The extract contains noticeable 
quantities of common salt, and organic salts of soda which leave 
sodium carbonate on ignition. 

Cocos nucifera, the Cocoa-nut Palm, also contains tannin in 
roots. 

CASUARIN^E 

Casuarina equisetifolia L. {laterifoUa Lam.) ; Filao bark. 
Reunion ; Tjamara laut, Java ; Casagha or Tinian Pine, Ceylon. 
Widely distributed in Southern Asia ; bark used for tanning and 
dyeing. Tannin gives blue-blacks with iron. Several other 
species very similar in structure and properties. (Von Hohnel.) 
Hooper found 11 to 18 per cent, of tannin. 



SALICACE^ (Willows) 

Most of the Willow tribe contain more or less iron-blueing and, 
presumably, pyrogaUol-tannin in the bark, and many have been 
long used for tanning, especially in Russia. The original " suede 
leather " was a combination tannage of the bark of Salix arenaria 
with alum and salt, and when this process was introduced in 



286 PRINCIPLES OF LEATHER MANUFACTURE 

France the commoner Salix caprcea was substituted, with addi- 
tions of oak bark and madder to compensate its deficiency in 
tannin and colour. Russia leather owes part of its odour to 
tannage with willow bark. 

Dekker gives Salix alba L., bark 9-4 per cent, tannin. 

Salix caprcea, bark i2-i per cent, tannin. 

Salix fragilis L., 9 per cent, tannin ; 5. Russeliana (a variety), 
12 per cent. 

Salix repens or arenaria L., Astrachan, 13 per cent. 

Poplars belong to the same natural order, and their bark, 
especially that of the Aspen {Popuhis tremula L.), has been used 
for tanning, but contains only about 3 per cent, tannin. 

MYRICACEiE 

Myrica Gale, Sweet Gale, or bog-myrtle, contains tannin. 

Myrica Comptonia or asplenifolia, U.S.A. ; " Sweet Fern." 
Covers millions of acres in Michigan. Yields 40 per cent, of 
" extract." Leaves 4 to 5 per cent., roots 4 to 6 per cent, tannin, 
according to season (Trimble). Has been much talked of, but 
in Professor Trimble's opinion is not likely to prove of much 
importance. 

Myrica nagi (Hind. Kaiphal), India, contains 13 to 27 per cent, 
of tannin in the bark, and a colouring matter, myricetin, identical 
with that of sumach. ^ Leather tanned with it is of a somewhat 
reddish colour, which is much brightened by sumaching, and con- 
verted into a pale yellow by treatment with alum. It promises 
to be a valuable tanning material. 

BETULACE^ 

Alnus glutinosa, Common Alder. Fr. Aulne ; Ger. Erie. 
Contains 16 to 20 per cent, iron-green tannin, with much red 
colouring matter ; old barks as low as 10 per cent. Colour 
develops during and after tannage. Used alone it gives a red, 
hard and brittle leather, but with galls, valonia, etc., it produces 
a satisfactory tannage. Its principal use is to furnish gunpowder- 
charcoal, and it is possible the bark might be obtained from 
powder-factories if the use of gunpowder is not superseded by 
nitro-compounds. (Von Hohnel.) The fruits have also been 
used in tanning. An important fuel-tree in high latitudes. 

Alnus niaritima, Hannoki, Japan ; and A. firma, Minibari. 

^ Perkin and Hummel, Trans. Chem. Soc, 1896, p. 1287. 



VEGETABLE TANNING MATERIALS 287 

Fruits (yashi) contain 25 per cent, tanning matter (iron-blueing), 
and little colouring matter. Used in Japan for dyeing and 
tanning. A. nepalensis and A. nitida used in India. Several 
other species of Alnus contain tannin. 

Betula alba, White or Common Birch. Fr. Bouleau hlanc ; 
Ger. Birke. Inner bark used in Scotland (in conjunction with 
larch for tanning sheep-skins), Norway, Russia, etc. It con- 
tains only 2 to 5 per cent, of iron-greening tannin, and muth 
fermentable sugar. The outer bark, " Naeve," covered with turf, 
is used in Norway for roofing, and is very durable and quite water- 
proof. By far the most important use of birch bark in tanning 
is to produce the birch-bark tar used to give scent and insect- 
resisting power to "Russia" leather {Youft ; Ger. Juchten). 
The outside bark consists of thin layers of cork, often white with 
a crystalline deposit of betulin, which when distilled yields the 
odorous oil. The distillation is a dry one, and tarry products 
accompany the true oil, and at first give a strong empyreumatic 
smell to the leather, which it loses by keeping, while the true 
" Russia " odour remains. This " ageing " may be hastened by 
hanging the leather in a hot stove. If the oil is distilled in a 
current of steam, or with petroleum ether, the tarry matter 
passes over, while the matter giving the true odour remains in 
the retort (p. 452). 

Betula lenta, American Black Birch. The bark and twigs 
distilled with water yield an essential oil, which is almost pure 
methyl salicylate, and largely substituted for oil of winter- 
green {Gaultheria procumbens), with which it is chemically 
identical. Used for perfumery, and as a rheumatism remedy. 
Often erroneously spoken of as the source of " Russia " oil. A 
mixture of a trace of wintergreen oil with sandal-wood oil con- 
siderably resembles the " Russia " scent (p. 453). Synthetic 
methyl salicylate is now made, but its odour, though pleasant, 
is different from that of the natural oil. The bark contains 
about 3 per cent, tannin. 

CUPULIFERiE 

Castanea vesca, True or Spanish Chestnut. Fr. Chdtaignier ; 
Ger. Kastanie. Abundant in Italy, South of France, and Corsica, 
where it forms great forests. Bark said to be nearly as strong 
in tannin as oak (up to 17 per cent., de Lof, more probably 6 to 
8 per cent.), but not much used in tanning. 

Wood only contains 3 to 6 per cent, tannin, but is the source 
of the valuable chestnut extract, first employed for dyeing, and 



288 PRINCIPLES OF LEATHER MANUFACTURE 

introduced as a tanning agent by Aime Koch. The strength 
of extract is of course very variable, even for the same density 
(see Chapter XXIV.), but it usually contains from 28 to 32 per 
cent, of tannin. The wood is a valuable timber, and is now 
grown in England for this purpose, as it is much quicker in 
growth than oak. 

The tannin gives blue-black with iron, but is not identical 
with either oak-bark or gall tannins, but apparently a mixture, 
or possibly a methylated derivative of the latter, and identical 
with oakwood tannin, or so nearly so as to be indistinguishable ; 
it may also be identical with divi tannin. Decolorised chestnut 
extracts sometimes mixed with quebracho and other materials 
are often sold as " oakwood " or " oak-bark " extracts. The 
extract gives a firm leather, with a good deal of bloom if used 
strong, and a more reddish tint than valonia. The extract often 
contains dark colouring matters, and the colour of leather tanned 
with it is readily darkened by traces of lime derived from cal- 
careous waters or imperfectly delimed hides. Like all wood 
extracts it tans rapidly, the colour penetrating first, and the tan 
following, but, according to Eitner, it does not, alone, make full 
or solid tannage, perhaps from want of acid-forming matters, but 
answers particularly well in combination with spruce-bark. It is 
largely used in England for sole leather in combination with 
valonia, myrobalans, and other materials. 

The higher the temperature of extraction, and the more 
colouring matter is contained in the extract in proportion 
to tannin matter, the greater is its viscosity. Much 
colouring matter remains undissolved if the extract is dis- 
solved in cold water, but there is, in addition, a loss of 
tanning power, the colouring matter being also capable of 
combining with hide, and it has in fact been used for tanning 
by dissolving it in solutions of borax or alkaline salts. By 
improved methods of manufacture the colouring matter has 
been much reduced. 

The chestnut is an important food tree, the nuts forming a 
considerable part of the food of the inhabitants of Corsica and 
Sardinia, and even of Italy. 

Oaks 

Almost all species of oak contain useful quantities of tannin 
in the bark, and probably in the wood. Most if not all oaks 
yield catechol-tannins with, probably, some mixture of ellagi- 
tannic acid. 



VEGETABLE TANNING MATERIALS 289 

Quercus robur, Common Oak. Fr. Chene ; Ger. Eiche. It is 
frequently separated into the two subspecies : — 

Quercus pedunculata. Commonest oak of lowlands, England, 
Ireland, and Scotland. Acorns in bunches or spikes on a stalk 
\ inch long, hence Ger. name, Stiel-Eiche. Leaves sessile or 
short -stalked. In favourable situations, said to yield about 2 per 
cent, more tannin than Q. sessiliflora, but this is doubtful. It is 
the commonest oak in Slavonia, and the source of commercial 
oakwood extract. 

Q. sessiliflora, Ger. Traubeneiche. Common in hilly districts, 
and scattered throughout the country. Acorns in bunch on the 
branch, or with very short stalk ; leaves on stalk ^ to i inch 
long. 

Of English barks, Sussex and Hampshire are considered the 
best, and contain up to 12 to 14 per cent, of tanning matter ; a 
coppice bark from Wastdale, Cumberland, is, however, recorded 
to have jnelded 19 per cent, tanning matter (Hellon). 

Probably each of the two varieties of oak gives best bark 
where it thrives best (von Hohnel). 

Belgian bark is sometimes equal to English, and contains 
10 to 12 per cent, tanning matter. Dutch bark as exported is 
generally inferior, and not cleaned ; Swedish is bright, but very 
poor. 

Oak-bark contains a tanning matter, quercitannic acid, giving 
green-blacks with iron-salts, and possibly containing both catechol 
and pyrogaUol groups, but its constitution is not fully under- 
stood. It yields both red anhydrides and ellagic acid ; and 
gallic acid has been obtained by the action of hydrochloric acid, 
though not by fermentation in the tannery. The tannin is not a 
glucoside, but the fact that a sugar, Isevulose, is also present in 
the bark has led some observers to erroneous conclusions regard- 
ing the constitution of the tannin. The unpurified infusion' of 
the bark of Q. robur gives a blue-black with iron-salts, from the 
presence of a colouring matter ; but those of most other oaks 
give green-blacks. 

Most tannin is contained in the living part of the bark. The 
5deld diminishes in trees over twenty-five years, and coppice 
barks, from absence of ross, are often strong, and also contain 
less colouring matter and more fermentable sugar. 

Warm and rich soils seem to jdeld the best barks. 

The brighter the colour of the fresh-cut " flesh," the better 
the bark. Dark brown inner side shows that bark has been 
exposed to rain, which deteriorates strength and colour ; but a 
very light colour is thought by some to indicate poorness in 

19 



290 PRINCIPLES OF LEATHER MANUFACTURE 

tannin. White lichen is said to be a mark of poor bark, and 
probably indicates a damp and unfavourable situation. 

Oaks are generally cut when the sap is rising (15th April to 
15th June), when the buds open and new soft cells begin to grow, 
for the bark is then more easily detached. 

Experiments in France have shown that the bark of timber 
felled at other seasons may be loosened by steaming, and it is 
said there is no practical loss of tannin. Superheated steam, 
produced in a small boiler in the woods, is used. 

The bark is peeled with tools of various forms, the branch 
and knotted places being loosened by beating with a mallet. 
The bark must be peeled immediately the tree has been felled. 

The peeled bark, in pieces up to 3 feet long, is laid on hurdles 
sloped in such a way that the rain runs off as much as possible, 
and in this way it is dried, but in wet seasons is much damaged. 
Bark so dried in the woods often retains 40 to 50 per cent, water, 
and must be stacked or stored so as to allow of further drying. 

English bark is sometimes sold in " long rind," and sometimes 
" hatched " or chopped in pieces about 4 inches long. Belgian 
and Dutch barks are generally hatched. Belgian tree bark is 
" cleaned " (and cleanings often mixed back with bulk), Dutch 
bark is not cleaned. Much sand and dirt is contained in most 
Continental bark : screenings of Belgian bark yielded a black 
liquor, and contained so much sand that they would not even 
burn ! 

Oak-bark extract is occasionally offered for sale, but is not 
usually genuine or of good quality, except that of the American 
chestnut -oak, Q. prinus, from which an excellent extract has been 
manufactured in the Alleghanies. Factitious extracts often 
contain myrobalans and quebracho. 

Oakwood contains only a very small percentage (from 2 to 4 
per cent.) of a tannin practically identical with that of chestnut, 
but different to that of oak-bark. It is stated by de Lof to reach 
9 to 14 per cent, in old heart-wood, but this is doubtful. The 
wood retains the tannin in its interior for a long time. Wood of 
a Roman bridge built at Mainz 55 B.C. is stated by de Lof to have 
still contained 1-14 per cent, tannin in a.d. 1881. A good deal 
of imitated oakwood extract is undoubtedly made from chestnut 
wood, and unfortunately no very satisfactory way of distin- 
guishing it is known, though oak-bark extract can be distinguished 
from oakwood by giving a precipitate at once, even in dilute 
solution, with bromine-water, while the wood gives a brown 
precipitate only after long standing. Precipitation by bromine- 
water is a general characteristic of catechol-tannins, and hence 



VEGETABLE TANNING MATERIALS 291 

a mixture of quebracho (a cheap catechol-tan) with chestnut 
would simulate oak-bark in this respect. If a few drops of the 
non-tannin solution or an alcoholic extract from the " total 
soluble " of extracts containing quebracho or other catechol- 
tannins be treated with concentrated sulphuric acid in a test- 



A 




Fig. 49. — Turkey Oak (Quercus cerris) 



tube a deep crimson will be produced, especially at the surface 
of the acid, which remains pink on dilution with water. With 
pyrogallol derivatives, such as genuine oakwood, a yellow or 
brown only is produced (J. Hughes). The test is very delicate. 
Another distinction is that bark extracts contain perceptible traces 
of manganese, but this cannot be relied on, as many wood extracts 
also contain some, probably derived from the twig and branch 
bark which is used along with the wood. Oakwood extract is 
now manufactured on an enormous scale in Slavonia, and is 
used both by sole and dressing leather farmers, chiefly to increase 



292 PRINCIPLES OF LEATHER MANUFACTURE 

the strength of the layer Hquors. The extract is also used to 
increase the weight of leather after tannage by mopping on the 
flesh side. All the best oakwood extract manufacturers con- 
tract to sell on analysis and colour estimation, and good Slavonian 
oakwood extract generallj^ contains 26 to 28 per cent, of tanning 




Fig. 50. — Cork Oak (Quercus suber). 



matter, giving a tintometer measurement of 4° to 5° red and 
20° to 25° yellow when a solution containing | per cent, of tanning 
matter is measured in a i cm. cell. For particulars of the manu- 
facture of concentrated extracts see p. 493 et seq. 

Q. cerris, Turkey Oak. Ger. Zerreiche. Common in Southern 
Europe ; a fine tree, but bark inferior to Q. robtir (fig. 49). 

Q. pubescens. Fr. Chine veht ; Ger. Weiss- or Schwarzeiche. 
In mountain districts and scattered in Southern Europe, about 
equal to Q. robur. 

Q. ilex, Evergreen Oak. Fr. Chine vert, Chine yense ; Ger. 



VEGETABLE TANNING MATERIALS 293 

Gruneiche, Steineiche ; Span, and Ital. Encina. South Europe, 
Algeria. Said to be somewhat stronger in tannin than common 
oak, yielding 5 to 11 per cent, of a rather darker-coloured tannin, 
but well adapted to sole leather. Good bark is smooth outside, 
without fissures, short in fracture. 

Q. siiber, Cork Oak. F. Chine liege ; Ital. Sughero, Suvero 
(figs. 50, 51). The outer bark is cork ; the interior bark contains 
12 to 15 percent, of tannin, which is redder than that of ordinary 




Fig. 51. — Section of Cork Oak, showing cork, inner bark and wood. 

oak. Trees at first produce an irregular cork, sold as " virgin 
cork " for ferneries, etc. After this is stripped, later growths 
are more uniform, and fit for use ; tanning bark is only obtained 
when the tree is cut down. Bark is rough but pale-coloured on 
both sides, and about i cm. thick ; interior like ordinary oak, 
but more strongly furrowed. Produced chiefly on Mediterranean 
coasts, and formerly largely used in Ireland. 

Q. pseudosuber , African Oak. Fr. Chine faux liege. Algeria. 
Not stronger than English oak, but with more colouring matter, 
hence strikes quickly through leather. Bark very thick. 

Q. Mirhecki. Fr. Chene Zeen. Algeria. Rapid growth. 
Bark contains 8 per cent, of tannin. 

Q. TozcB. Fr. Chine tauzin. Pyrenees and S. France. Bark 
contains 14 per cent, of tannin. 



294 PRINCIPLES OF LEATHER MANUFACTURE 

Q. coccifera, Kermes Oak. Fr. Kermes, Garouillc (fig. 52). 
South Europe and Algeria. Root bark is called " rusque " or 
" garouille " ; averages 10 to 18 per cent, tannin, but trunk bark 
does not exceed 11 per cent. This tree is the food of the kermes 
insect, used for dyeing scarlet before the introduction of cochineal. 




Fig. 52. — Kermes Oak {Quercus coccifera). 

Garouille is principally used in the South of France, giving a 
firm sole leather of a disagreeable odour and dark brown 
colour. 

Q. Mgilops (and probably other species — Q. macrolepis, grcBca, 
Ungeri, coccifera), Valonia. Fr. Valonee ; Ger. Valonea, Acker- 
doppen, Orientalische Knoppern. Best Smyrna contains up to 
40 per cent., Greek 19 to 30 per cent., Candia valonias up to 41 
per cent., and Caramanian (probably not Q. Mgilops) 17 to 22 
per cent, of tannins, which are at least principally pyrogallol 
derivatives, and which give blue-blacks with iron, no precipitate 



VEGETABLE TANNING MATERIALS 295 

with bromine-water, and which deposit a great deal of bloom, 
consisting of ellagic acid. 

Q. Mgilops (fig. 53) is said to be most abundant in the high- 
lands of Morea, Roumelia, Greek Archipelago, Asia Minor, and 
Palestine, while macrolepis forms large forests in many parts of 




Fig. 53. — Valonia Oak {Q. Mgilops). 



Greece, and especially on the lower slopes of Mount Taygetos. 
In Asia Minor the fruit ripens in July to August, when the trees 
are beaten and acorns left on the ground to dry. They are after- 
wards gathered, and carried on camels to stores in the towns, 
and thence by camel and rail to Smyrna, where they are placed 
in heaps 5 to 6 feet deep in large airy stores, and allowed to 
ferment and heat for some weeks, when the acorn, which contains 
but little tannin, contracts and falls from the cup, and is used for 
feeding pigs. This fermentation is risky, and if carried too far 



296 PRINCIPLES OF LEATHER MANUFACTURE 

the cups become dark coloured and damaged. The acorn con- 
tains a considerable amount of fermentable sugar. 

When ready for shipment the valonia is hand-picked, the 
largest and finest cups (prima) going to Trieste, the second 
selection to England (Inglese), and the remainder, known as 
" natural," also coming largely to England. The " Inglese," 
although inferior in appearance to the very large selected cups, 
is of course less costly, and gives an equal yield of tannin. 

In 1887 Smyrna exported about 23,000 tons to England 
and 16,000 tons elsewhere, principally to Austria, Germany, and 
Italy. The largest known crop is stated at 70,000 tons in Asia 
Minor and 14,000 in Greece, but the average yield is considerably 
less than this. 

The beard contains considerably more tannin than the cups, 
sometimes over 40 per cent. It is often sold separately at the 
same or a lower price, and in Smyrna is known by the Turkish 
name tirnac (Ital. trillo). 

In Greece the best valonia is collected (in April ?) before the 
cup is matured and whilst it still encloses the acorn, and is known 
as chamada (It. camata and camatina). The colour of these' 
kinds is excellent and the percentage of tannin high. Mainly 
used by dyers, but often worth attention for tanning where 
colour is important. In camatina the acorn is completely 
covered in the cup, while in camata it is partly exposed. 

The next quality, rhabdisto, is beaten down by sticks in Sep- 
tember to October (hence name), while after the first rains the 
fruit falls and turns black, and is called charcala. It contains 
but little tannin, and is not generally collected. 

Sometimes valonia is attacked by a sort of honeydew, prob- 
ably caused by an aphis, which renders it very sticky, and 
perhaps more liable to heat, but does not in itself damage it? 
tanning properties. 

The lighter the colour, the heavier the weight, and the thicker 
the scales of the beard, the better the quality usually proves, 
but analysis is the best guide. Caramanian valonia is very 
inferior. 

The tannin contained in valonia is especially valuable in the 
manufacture of sole leather. It deposits much bloom, and if 
used as a dusting material has the characteristic of making the 
leather solid and compact, but leaves the grain somewhat rough 
and hard to work. In mixture with gambler and other materials 
it is an excellent tannage for dressing leather, and with proper 
management deposits little or no bloom (cp. p. 364). 

Q. infectoria (fig. 54) is the source of the " Turkish " or Aleppo 



VEGETABLE TANNING MATERIALS 



''97 



galls. Galls are caused by insects, principally of the genus 
Cynips, or gall-wasps, which lay their eggs in different parts of 
plants, and in some way cause an abnormal growth of the bud, 
leaf, or other part. The gall-wasps affecting the different oaks 
are shown in the figures. 

Aleppo galls are developed from the young shoot of the oak. 




Fig. 54. — Gall Oak {Q. infectoria) . 



are best before the insect has escaped, and contain in this stage 
up to 50 or 60 per cent, of gallotannic acid. When the insect 
has developed and escaped the galls are of course perforated, 
much lighter, and more porous. These galls and those of 
Rhus semialata are the principal sources of the tannin of 
commerce. 

The Q. infectoria also bears a large gall like an apple, " Apples 
of Sodom," or " rove," caused by a different insect. This, in a 
crushed condition, has been somewhat largely used as a tanning 
material, and contains 24 to 34 per cent, gallotannic acid. 



298 PRINCIPLES OF LEATHER MANUFACTURE 

English oaks have several species of galls and oak-apples, 
but they do not seem to be of much value for tanning. 

Knoppern are galls produced on the immature acorns of various 
species of oaks, principally Q. cerris in Hungary, and were 
formerly largely used there for tanning, as they contain up to 
35 per cent, gallotannic acid. They are now less abundant, and 
have been largely replaced by valonia, sometimes called oriental- 
ische Knoppern. Like all purely gallotannic materials, they 
naturally give a soft and porous tannage, ill-adapted for sole 
leather purposes, which has led to the Austrian practice of 
dr5n.ng, or rather stewing, the leather in very hot and damp 
stoves, which make it hard and brittle. 

Chinese and Japanese galls are the product of the action of 
an aphis on a species of sumach, and will be mentioned again 
under sumachs {Rhus). 

Djaft, dchift, jift, or jaft is a material apparently of Eastern 
origin, and said to be derived from an oak of Kurdistan. Dark 
red scales or fragments, origin uncertain, very astringent and 
darkish tannage ; liquor when spilt dries whitish, apparently from 
crystallisation of some sort. It contains a large amount of 
tannin. It appears very irregularly in commerce, and the writer 
would be glad to obtain further samples and details of origin. 
He once used 6 or 7 tons successfully in sole leather tannage. 
It has also been attributed to a shrub allied to the Ccssalpinias 

(P- 327)- 

The most important American oaks are : Q. prinus {castanea, 
monticola), the Chestnut or Rock Oak (fig. 55). About equal to 
our oak in strength, bark very thick, and infusion strongly fluor- 
escent, especially in presence of ammonia. Source of chestnut- 
oak extract. The most important tanning oak-bark of the 
United States. 

Q. alba, or " white oak," is perhaps the most widely distributed 
and abundant of any of the American oaks, and very closely 
resembles the European Q. robur. 

Q. tinctoria or nigra, Black or Quercitron Oak. Poor as a 
tanning material, but used for dyeing yellow and for modifying 
the colour of hemlock tannages. The dyestuff, quercetin, is 
closely alHed to that of fustic, and gives yellows with alum and 
tin mordants. 

A good deal of information is given by Trimble ^ on American 
oaks and other tanning materials. 

Q. pachyphylla. Acorn cups used, very similar in appearance 
to Greek valonia. Tree grows in hilly districts of N. India. 
^ The Tannins, vol. ii. Lippincott, Philadelphia, 1894. 



VEGETABLE TANNING MATERIALS 299 

Tannin gives strong but good colour. Analyses by Mr H. 
Brumwell show 13 to 15 per cent, tannin and 9 to 13 per cent, 
non-tannin in dry cups. Acorns (dry) showed 2-0 per cent, 
tannin and 5-6 per cent, non-tannin. The .usual proportion of 
cups to acorns is about 70 : 30. The leaves and bark contain 




Fig. 55. — Chestnut Oak {Q. prinus). 



considerable tannin. According to Pilgrim 10 per cent, in the 
former and 12 to 13 per cent, in the latter. 

Q. fenestrata. N. India. Mature bark contains 15-8 per cent, 
tannin and 8-4 per cent, non-tannin (Pilgrim, calc. on dry 
material), and yields liquor of very light colour. It is suggested 
as suitable for extract manufacture. 

Q. lineata. N. India. Twig bark contains 10-5 per cent, 
tannin and ii-o per cent, non-tannin (Pilgrim, dry material), and 
is slightly richer than mature bark. Colour good. 
. Q. lamellosa. N. India. Leaves, twig bark, and mature bark 
contain 8 to 10 per cent, of tannin and 13 to 18 per cent, non- 



300 PRINCIPLES OF LEATHER MANUFACTURE 

tannin (Pilgrim, dry material). The liquor has a strong but 
not red colour. 

Other important Indian oaks are Q. glauca and Q. incana ; 
bark of last said to j^eld 22 per cent, of tannin. 

In addition to these, Dekker gives a large number of other oaks 
containing tannins, but mostly in quite small quantities, and 
rarely so much as 12 per cent. Q. rubra (common red oak of 
N. America) has twig-galls, in which Trimble found 34-8 per 
cent, tannin, but only found 4 per cent, in the bark, though 
Eitner found 16 per cent. 

POLYGONACE^, Docks 

The roots of most members of this family contain tannin, often 
in considerable quantity. 

Riimex hymenosepalum, Canaigre, Gonagra (Cana agria). Red 
Dock, wild pie-plant (fig. 56). Common in sandy alluvial plains 
of Mexico and Texas, and considerably resembling rhubarb. Its 
tuberous roots resemble those of the dahlia, and contain, when 
air-dried, 25 to 30 per cent, of a catechol-tannin, probably allied 
to that of mimosa, but of a much paler and yellower colour. 
Undried, the roots contain about 68 per cent, of water and only 
8 per cent, of tannin. When well harvested by slicing thin and 
rapidly drying it gives leather a bright orange colour and, it is 
said, considerable weight and firmness, and is thus specially 
suitable for use in retanning and finishing light goods and harness 
leather. Besides tannin, the root contains a yellow colouring 
matter and about 8 per cent, of starch, of which the granules 
are very variable in form and size, but mostly oval or elongated, 
and do not stain readily with iodine till they have been well 
washed or treated with dilute sulphuric acid. Both the starch 
and tannin are contained in large and somewhat thin-walled 
cells, and the sliced material is easily extracted at low tempera- 
tures. Greater heat gelatinises the starch and extracts a darker 
colour. The best temperature for extraction is between 30° and 
50° C. (see p. 412). The large content of starch and the cost of 
culture and harvesting has told against the use of this plant. 
For a drum-tannage on Dr Turnbull's principles, however, the 
presence of starch would be no disadvantage [cp. p. 573). 

The root is most readily grown from tubers or portions includ- 
ing the crown, as the plant seeds sparingly. Sandy soils, subject 
to inundation or irrigation, seem best suited to its culture. In 
California and Arizona the growth begins in October or November 
with the winter rains, blooming about the end of January, while 




Fig. 56— Canaigre {Rumex hymenosepalum) . New Commercial Drugs and Plants, T. Christy. 



302 PRINCIPLES OF LEATHER MANUFACTURE 

the leaves die down in May and the roots remain dormant during 
the summer. It is not important at what time the roots are 
harvested, and they seem to improve in percentage of tannin up 
to the second year, after which they become darker and deteriorate. 

The harvested crop should be sliced into thin pieces and 
rapidly dried at a low temperature, or, still better, converted at 
once into extract. This is already done on a considerable scale 
at Deming, New Mexico. The residue after extraction is used 
in America as cattle-food ; and might no doubt be also applied 
to the production of alcohol. 

Planting takes place in autumn, in rows, say 30 inches apart, 
with 10 inches between each root. Roots for " seed " should be 
kept in the ground or stored in dry sand. This should yield a 
crop of 10 tons per acre in an average season. 

References. — Report U.S. Commissioner of Agriculture, 1878, 
pp. 119 et seq. ; Trimble, American Journ. of Pharmacy, p. 395, 
1889 ; Canaigre, Buh. No. 7, Arizona Agr. Expt. Station, 1893 ; 
Canaigre or Tanner's Dock, Bull. No. 105, University of Cali- 
fornia, Berkeley, Cal. ; Canaigre Tannin, Trimble and Peacock, 
Philadelphia, 1893 ; Report to the German Leather Trades Associa- 
tion, by V. Schroeder, 1894 ; II Canaigre, E. Andrieis, Turin, 
1899. 

Rumex maritima. Central Europe, England, Ireland. It is 
said by de Lof to be found in CaUfornia, where it is used by the 
Indians for tanning ; but he probably confounds it with canaigre. 
De Lof found its roots, wet, to contain 6 per cent., and after 
drying 22 per cent, of tannin, together with starch and an acid 
allied to malic. 

Several English docks contain tannin ; the writer had a sample 
of leather tanned with dock-root (very possibly R. aquaticus), 
many years old, but still soft and close in texture, and of excellent 
quality. 

Polygonum amphibium. Said to grow on thousands of acres (?) 
on the lower Missouri. Roots contain 22 per cent., branches 
17 per cent, of tannin. P. amphibium is a common English and 
European plant, with spikes of pink flowers, growing in marshes 
and ponds. Probably this is the Polygonum analysed by Fraas, 
who found 20 to 26 per cent, tannin. 

Polygonum Bistorta. Common in damp places in England. 
Bistort, Snakeweed, called " Eastermer giants " in Cumberland, 
where the young leaves are used for making herb-puddings. 
Fraas found 16 to 21 per cent, tannin in the roots. 

Other species are known to contain much tannin. Perkin 
found a red colouring matter in P. cuspidatum, a native of India 



VEGETABLE TANNING MATERIALS 303 

and China, commonly grown in gardens as a foliage plant {Journ. 
Chem. Soc, 1895, p. 1084). P. tinctorium, used as a source of 
indigo in China and Japan. 

Coccoloha tivifera, Seaside Grape of West Indies ; source of 
West Indian kino. Whole plant rich in tannin. 

Dekker also mentions Coccoloha crescentifceolia (Cham, and 
Schl.) of Brazil, which gives an astringent extract. A number 
of other species of Polygonum and Rumex contain tannins in 
unimportant quantities. 



LAURACEiE, Bay Family 

Persea, or Laurus lingue. Bark used in Chili for tanning 
Valdivia leather. (According to Arata, Laurus caustica.) A tree 
25 to 30 feet high and 2 feet in circumference. Bark rough 
outside and whitish, with an aromatic smeU and taste, brittle 
and easily ground, contains 17 to 19 per cent, of a catechol- 
phloroglucol tannin, greening iron-salts {Journ. Chem. Soc, 1881, 
p. 600) ; 22-0 per cent, tannin, 8-3 per cent, non-tannins 
(Pollak, Coll., 17, 1918, p. 3). A large number of heavy hides are 
tanned yearly with this bark in Valdivia and district, and mostly 
sent to Hamburg. The hides are thick and scarcely tanned 
through, colour fair, leather soft and porous. 

Persea Meyerina N. and Laurus Pneumo. Said to be also 
used in Chili. 

According to Eitner, both these barks contain considerable 
quantities of starch. Many others of this family contain tannin, 
but mostly in small quantity. 

MYRSINACE.ZE 

Myrsine gardneriana D.C. " Capororocca," Brazil, Rio 
Grande do Sul ; bark largely used for tanning. ^ 

PROTEACE^ 

Banksia serrata. Heath Honeysuckle. Australia. Specimen 
examined contained 11 per cent, tannin ; according to Maiden 
it reaches 23 per cent. 

Banksia integrifolia. Queensland. Bark contains 11 per cent, 
tannin. Maiden found io-8 per cent, tannin. 

Grevillia striata. Australia. Bark contains 18 per cent, 
tannin, yielding reds. 



304 PRINCIPLES OF LEATHER MANUFACTURE 

Leucospermwm conocarpitm. Kruppelboom. Knotted Tree. 
Cape of Good Hope. Said by de Lof to contain 22 per cent, of 
tannin ; but a specimen examined by the Author yielded 10-9 
per cent, on analysis. 

Protea meUifera. Sugarbush. Suikerhosch. Cape of Good 
Hope. Bark contains 25 per cent, tannin, according to de Lof ; 
but Palmer found i8-8 per cent. Dekker only states 3 per cent. 

Protea grandiflora. Waagenhoom. Cape. Contains 25 per 
cent, tannin (de Lof) ; .15-9 per cent. (Palmer) ; 15-6 per cent. 
(Procter). 

Protea speciosa. Cape of Good Hope. Used for tanning, 

Leucadendron argente^im, Silver Tree, Silverhoom, Wittehoom. 
Cape of Good Hope. Bark said to contain 16 per cent, tannin 
(de Lof) ; a specimen examined by the Author yielded 9-2 per 
cent. 

Brahium stellatifolhim, Wilde Amandelhoom, Wild Almond. 
Cape of Good Hope. Astringent. 

SANTALACEiE 

Osyris compressa {Fusanus conipressus, Colpoon compressum, 
Thesium Colpoon), " Cape Sumach," " Pruim Bast," leaves and 
bark, Cape of Good Hope. Leaves contain about 23 per cent. 
(Perkin), 17 per cent. (Maiden) of tannin, and form a useful 
substitute for sumach ; but the tannin is not identical, and is 
of the catechol class, resembling gambler. 

0. arborea. Northern India. Leaves rich in tannin ; 20 per 
cent. ; contains quercetin (Perkin). 

0. ahyssinica. Somahland. The leaf contains 23 to 25 per 
cent, tannin (Watter). 

Fusanus aaiminatus [SantaUim acuminatum), " Quandony." 
Austraha. Eighteen to 19 per cent, dark-coloured tannin. 

Exocarpus cupressiformis. Austraha. Bark contains 15 per 
cent, tannin (Maiden). 

DAPHNOID^, Spurge Laurels 

Daphne Cnidium L.," Garou." Algeria. Used for dyeing and 
tanning. 

PLUMBAGINiE 

Phimhago Europea, Leadwort. Fr. Dentelaire. A garden 
plant in England, native in France; contains much tannin, 
especially in the root -bark. 



VEGETABLE TANNING MATERIALS 305 

Statice coriaria, Marsh Rosemary. South of Russia. Roots 
up to 3 metres long and 2 to 12 cm. thick ; used by Kalmucks 
for tanning sheep-skins ; contain 22 per cent, of tannin (de Lof). 
Dekker gives ~20 to 22 per cent. 

5. gmelini W. South Russia. Used as above. 

5. latifolia. Caucasus and Middle Asia. Strongly astringent. 

S. scoparia Pall. Siberia. Strongly astringent. 

S. limomtm L., Sea Lavender. Coasts and salt marshes 
of Europe and America. Richer in tannin than 5. coriaria ; 
used in France, Spain, and Portugal. 

Several other species contain tannin. These plants are allied 
to " Thrift " [Armeria), which is also astringent. 



MALPIGHIACEiE 

Byrsonima spicata (Rich.). S. America, Antilles, " Tamwood." 
Bark contains up to 43-5 per cent., tannin. 

B. coriacea. Jamaica, " Golden Spoon." 

B. chrysophylla, etc. 

B. crassifolia a.B.K. Cuba. Contains much tannin in bark 
and unripe fruit. 

B. cydonicB folia, var. chiquitensis (Juss.). Bolivia. Native 
name " mureci." Bark contains 20 per cent, tannin. 

Malpighia punici folia. Nicaragua, " Nancite " ; " Mangrutta." 
Bark contains 2crto 30 per cent, of light-coloured tannin. 

M.faginea (Sw.). Mexico, " Nance." Bark up to 26 per cent. 



POLYGALACE.E, Milkwort Family 

Krameria triandria, Rhatany, Peru. 

The root is used in medicine, and is stated to contain 40 per 
cent, of tannin. 

Wittstein found only 20 per cent, of an iron-greening catechol- 
phloroglucol tannin allied to tormentil tannin in the root -bark, 
the only active part of root. 



EUPHORBIACE^ 

Cleistanthus collinus, " Kodarsi," Deccan. Bark stated to 
contain 33 per cent, of tannin. 

Phyllanthus emhlica. (Aonla, Amla). India. Twig bark, 
mature bark, leaves, and immature fruit (embhc myrobalans) 

20 



3o6 PRINCIPLES OF LEATHER MANUFACTURE 

all contain considerable tannin. Analyses by Pilgrim and others 
as follows : — 

Leaves. Twig Bark. Fruit, without Stone. 

Per cent. Per cent. Per cent. 

Tannin 23 to 28 19 to 24 26 to 35 

Non-tannins 14 „ 19 10 ,, 17 37 .. 40 

Twig bark gives smooth grain, steady swelling of hides during 
tannage. Reddish colour when used alone. Very useful 
material. 

P. disiichus and nepalensis both yield tanning barks. 

ANACARDIACE^ 

Schinopsis halanscB (Engl.), also named Quebrachia Lorentzii 
and Loxopteryngmm Lorentzii. Span. Quebracho Colorado. South 
America, especially Argentine Republic ; the highest proportion 
of tannin occurring in the wood from Gran Chaco district. Wood 
contains on an average about 25 to 28 per cent, of a red, difficultly- 
soluble tannin, yielding " reds," and containing catechol and 
phloroglucol. The tannin is not very soluble in water, and hence 
can only be used in weak or warm liquors, but is very astringent, 
and gives a firm, reddish leather. The wood also contains a 
catechin ^ and a colouring matter, fustin, identical with that of 
" young fustic." It is imported into England, and more largely 
to Havre and Hamburg, in logs, which are there chipped like 
logwood, and either used direct for tanning or made into extract. 
A very cheap tan. With alum it gives a yellow colour. The 
extract usually dissolves to a fawn-coloured turbid solution. 
Many quebracho extracts are now made completely soluble by 
treatment with alkaline bisulphites {cp. p. 404), or by treatment 
with alkali, which is afterwards neutralised. The so-called 
" insoluble " matter is really a sparingly soluble tannin, and if 
agitated with hide will tan it completely. 

The Spanish word " quebracho " means " axe-breaker," and is 
applied to many hard woods. The wood of 5. halansce has a 
specific gravity of 1-27 to 1-38, and is consequently much heavier 
than water. It is much used for railway sleepers, for which it is 
very durable. The sapwood only contains 3 to 4 per cent, and 
the bark 6 to 8 per cent, of tannin. 

The annua] cut is said to be much less than the annual growth. 
It is common in Chili and Paraguay, as well as in the Argentine. 

1 See P. Arata, Journ. Chem. Soc, 1878, A, p. 986 ; 1881, A, p. 1152 ; 
and Parkin and Gunnell, Trans. Chem, Soc, 1896, 1303, 



VEGETABLE TANNING MATERIALS 307 



(" Rep. on Tanning Materials of Latin America," T. H. Moran, 

Special Agents' Bulletins, No. 165, from J.A.L.C.A., 14, 1918, 

p. 441.) 

Aspidospermum quebracho (quebracho bianco) belongs to the 

ApocynacecE. It contains but 

little tannin, but is valuable for 

an alkaloid, aspidospermin. 
Schinus mollis, "Molle," Buenos 

Ayres. Leaves only used ; said 

to contain 19 per cent, tannin. 
5. Aroeira, Brazil. Said to 

contain 14 per cent, tannin. 
Pistacia lenfiscns, Ital. Pistacio, 

Fr. Lentisque. Sicilj^ Cyprus, 

Algeria. Small myrtle-like leaves 

contain from 12 to 19 per cent. 

of a catechol-tannin, and are 

very largely used in the adul- 
teration of sumach. Leather 

tanned with sumach adulterated 

with this material darkens and 

reddens on exposure to light and 

air, and for this reason its use in 

many cases is decidedly injurious. 

In Cyprus and the East it is 
known as " Skens," Ital. Schinia, 

Fr. Poudre de Lentisque, in England often called Cyprus sumach 

{cp. p. 309). 

P. orientalis, terebinthus, vera, etc., India, Mediterranean. Yields 
various aphis galls, 30 to 40 per cent, tannin. A sample of galls 
of Pistacia vera, " Gool-i-pista," India, examined in the Author's 
laboratory, contained 30 per cent, of a light-coloured tannin. 

Rhus coriaria, Sicilian sumach. Ital. Somacco (fig. 57). A 
shrubby bush, of which leaves and small twigs are used. 

Mostly propagated by suckers from older plants, which are 
planted in rows about 2 feet apart in early spring, and pruned 
to 6 to 8 inches. Bushes begin to bear the year after planting, 
though the strength is not so good as from more mature plants. 
Cropping is either by pruning off shoots, or gathering leaves by 
hand ; in the latter case shrubs are pruned in winter. The leaves 
are dried either in the fields or on covered threshing floors, 
where they are afterwards separated from the stems by beating. 
Some is exported in this state as " leaf "or " baling " sumach, 
but most is ground to fine powder under edge-runners. " Venti- 




FiG. 57. — Sicilian Sumach 

{Rhus coriaria). 



3o8 PRINCIPLES OF LEATHER MANUFACTURE 

lated " sumach is winnowed to remove dust and sand, which often 
contains iron. " Mascolino " is the best sumach from Palermo 
and district ; " feminella" consists of weaker sorts from other 
parts, and is generally used for mixing. 

The different varieties of sumach are classed as follows : — 

Relative 
Market Value. 
Sumach for baling ..... 2-5 

,, for grinding . . . . . 2-3 

,, from yearling plants , . . 1-5 

,, from ends of branches collected in 

autumn . . . . i-o 

To prepare these different grades for ultimate consumption 
they are ground in mills similar to those employed for crushing 
olives, that is, in which two large stone wheels follow each other, 
revolving upon a circular bed, the whole construction being 
similar to the Spanish or Mexican arrastre. The sumach thus 
pulverised is passed through bolting-screens to separate the 
finer from the coarser particles. 

After the sumach leaf has been subjected to the first process of 
trituration, the coarse remaining portions are re-ground and the 
product added to that which has been already obtained. The 
still unpulverised residue, known as peduzzo, is sifted, and the 
coarser and ungrindable parts are used as fuel, while the finer 
are mixed with the partially-ground, small, leaf-bearing branches 
(gambuzza, gammuzza), and ground again. 

Palermo is the principal seat of the sumach trade. The 
material is generally bought from the small growers by middle- 
men, who hold it till market conditions are favourable. The 
quotations are always in tari of 42-5 centimes per cantar of 
79-342 kilos,, which are obsolete even in Sicily, and have to be 
reckoned into lire (francs) and kilos. Consequently i tari per 
cantar equals 0-53565 lira per 100 kilos. 

In 1894, the prices delivered at the mills were about 41 to 42 
tari for mascolino, 37 to 38 tari for feminella, 14 to 18 tari for 
brusca, and 10 tari for stinco per cantar, the lira being worth 
about 9d.^ 

Sumach has been introduced into Australia, and is said to 
thrive well in the dry plains of the Wimmera district. 

Sumach often contains much sand, and sometimes particles 
of magnetic iron ore, which cause black stains, and may be 
collected by a magnet, and which dissolve in dilute hydrochloric 
^ Cf Kew Bulletin, No. 107, pp. 293-6. 



VEGETABLE TANNING MATERIALS 309 

acid without evolution of hydrogen to a yellow solution. Metallic 
iron, which is also attracted by the magnet, dissolves in hydro- 
chloric acid with effervescence to a colourless or green solution. 

Good sumach contains at least 25 to 27 per cent, of tannin. The 
Author has analysed samples of undoubted genuineness contain- 
ing as much as 32 per cent, of a tannin, allied to gallotannic, 
with some ellagitannic acid, and a colouring matter (myricetin) 
identical with that of Myrica nagi (p. 286), which gives yellows 
with alumina and tin mordants, and is fugitive to light. 

Sumach is the best tanning material known for pale colour 
and soft tannage, and is hence used for moroccos, roans, skivers, 
etc., and also for brightening leathers of darker tannages, such 
as mimosa, gambler, the colouring matters of which warm sumach 
liquors seem able to dissolve. 

In the report of the Society of Arts Committee on bookbinding 
leathers,^ it is stated on abundant evidence that sumach-tanned 
leathers are less affected by light and gas-fumes, and less liable to 
decay, than those of any other known tannage. 

Sumach is frequently adulterated with the ground leaves and 
twigs of Pistacia lentiscus ^ (" schinia " or " skens "), Coriaria 
myrtifolia (" stinco "), Tamarix afncana {" brusca "), Ailantus 
glandulosa, Vitis vinifera (leaf of the common grape vine), Cistus 
salvifolius (L.), and some other species of the Rhus family, but 
Pistacia lentiscus is used to a much larger extent than any of the 
others. Pistacia, coriaria, and tamarix all contain considerable 
quantities of tannin, though less than genuine sumach, and of a 
different chemical constitution. 

The most satisfactory method of detecting these adulterants 
is by microscopic examination, none of the chemical methods 
proposed being very satisfactory ; though, as many of the added 
matters contain catechol-tannins, while those of sumach are purely 
pyrogallol derivatives, the method proposed by Hughes for the 
detection of quebracho in oakwood by the reaction of concen- 
trated sulphuric acid might render good service, and any sumach 
infusion which was rendered turbid by bromine-water would at 
least be open to grave suspicion. 

The most important work on the microscopic structure of the 
tissues of sumach and its adulterants was done by Andreasch, 
when during the later stages of his last illness he was obliged to 
winter in Sicily.^ His work will well repay study, but unfortu- 

1 Soc. Arts Journ., 1901, p. 14. 

- Adulteration has a good deal lessened, since it has been easily detected 
by the microscope. 
^ Sicilianischer Sumach und seine Verfalschung, Wien, 1S98. 



310 PRINCIPLES OF LEATHER MANUFACTURE 



nately does not admit of useful abstraction here. A very useful 
investigation was also made in the Author's laboratory by Messrs 
M. C. Lamb and W. H. Harrison ^ as regards the treatment and 
examination of the leaf-cuticles, which renders the detection of 
mixture comparatively easy. For details, the original memoir 
must be consulted, but if the suspected sumach be gently warmed 

for a few minutes with strong 
' nitric acid, its more delicate 
leaf structure is entirely de- 
stroyed, and after washing and 
neutralising with sodium car- 
bonate the strong cuticles of 
the leaves of the more common 
adulterants, " schinia " {Pistacia 
lentiscus), " stinco " {Coriaria 
myrtifolia) , " brusca " {Tamarix 
africana), and Ailantus glan- 
dulosa, are uninjured, and easily 
recognised. Examination is 
rendered easier by dyeing the 
cuticles, safranine, acid green, 
Bismarck brown, and naphthol 
yellow being suitable for the 
purpose. Mr Lamb's photo- 
graphs of the cuticles are re- 
produced in figs. 59-66, but, if 
possible, it is most satisfactory 
to compare the suspected sample 
direct with known specimens of 
the adulterants. 
R. glabra, Southern States, U.S.A. (fig. 58). Very largely 
used in the States to take the place of Sicilian sumach. A 
sample collected by the late Professor Trimble, and analysed in 
the Leather Industries Laboratory, contained 25 per cent, of 
tannin, and produced a leather of very much darker colour than 
Sicilian, but this may be largely due to carelessness in its 
harvesting. 

R. typhina, " staghorn " or Virginia sumach, contains 10 to 18 
per cent, of tannin. A sample from same source as above 
contained 13 per cent. 

R. cotonoides, U.S.A. The analysis of a sample of this material 

^ " Sumach and the Microscopic Detection of its Adulterants," Jouvn. 
Soc. Dyers and Colorists, March 1899. Cp. also Leather Chemists' Pocket 
Book, p. 202, 




Fig. 58. — American Sumach 
{Rhus glabra) . 



VEGETABLE TANNING MATERIALS 311 




Fig. 59. — Ailaiitus glandulosa. ■ Fig. 6o. — Coriana myvtifolia. 




Fig. 61. — Colpoon coinpressa. Fig. 62. — Rhus cotinus. 



312 PRINCIPLES OF LEATHER MANUFACTURE 

gave 21 per cent, of tanning matter, and leather tanned with it 
was almost equal in colour to that from R. glabra. 

Other sorts found in States : R. semialata (5 per cent, tannin) ; 
R. aromatica (13 per cent, tannin) ; R. metopium (8 per cent.) ; 
R. copallina, R. pumila, R. canadensis ; R. toxicodendron is the 
well-known " poison ivy," a climbing plant which causes a severe 
and irritating eruption if touched. 

R. glabra and R. copallina are chiefly recommended for extended 
cultivation in the United States. 

In Virginia the leaves are collected and cured by the country 
people, and sold and delivered to owners of mills for grinding. 
Their particular object being to secure the largest possible quan- 
tity of product at the lowest cost, httle attention is given to the 
quahty obtained or the manner of collecting. The most intelh- 
gent dealers in the raw material urge upon collectors to observe 
the following particulars : — To ensure a maximum value for 
tanning purposes, the leaf should be taken when full of sap, 
before it has turned red, has begun to wither, or has been affected 
by frost. Either the leaf -bearing stems may be stripped off, or 
the entire stalk may be cut away, and the leaves upon it allowed 
to wither before being carried to the drying-shed ; but care must 
be observed that they are neither scorched nor bleached by the 
sun. When wilted, they are carried to a covered place, and 
spread upon open shelving or racks to dry, avoiding the deposit 
in any one place of a quantity so great as to endanger the quality 
of the product by overheating and fermentation. Sumach should 
be allowed to remain in the drying-house for at least one month 
before sending to the market ; in case of bad weather, a longer 
period may be required. When ready for packing for shipment 
it should be perfectly dry and very brittle, otherwise it is hkely 
to suffer injury in warehouses from heating and fermentation. 

Buyers of sumach leaves for grinding depend largely upon 
colour for the determination of the value ; the leaves should, 
therefore, when ready for market, present a bright-green colour, 
which is evidence that they have suffered neither from rain after 
being gathered, nor from heating during the process of drying. 
Leaves having a mouldy odour or appearance are rejected. The 
Virginian crop reaches 7000 to 8000 tons, and is collected at any 
time between ist July and the appearance of frost. 

There is an important difference in the value of the European 
and American products. The proportion of tannic acid in the 
latter is generally lower than that found in the former, which 
is much preferred by tanners and dyers. By using Sicilian 
sumach it is possible to make the finest white leathers, while by 



VEGETABLE TANNING MATERIALS 313 




Fig. 63. — Pistacia lentiscus. ■ Fig. 6^. — Rhus metopium. 




Fig. 65. — Rhus coriaria. Fig. 66. — Tamarix africana. 



314 



PRINCIPLES OF LEATHER MANUFACTURE 



the employment of the American product the leather has a 
disagreeable yellow or dark colour, apparently due to a colour- 
ing matter which exists in larger quantity in the American 
variety than in the Sicilian. 

Experiments upon the presence of colouring matters made 
by treating an infusion of sumach with a solution of gelatine 
gave the following results : — 

Virginia, mixed, collected in June, gave A nearly white pre- 
cipitate. 

...., ., ., July, : 



,, R. copallina 

,, R. glabra 

Fredericksburg, mixed 

Sicilian 



A decidedly yellow- 
ish-white preci- 
pitate. 
August,, A dirty-yellow pre- 
cipitate. 
„ „ A very dirty-white 
precipitate. 
„ „ A dirty - yellow 

precipitate. 
,, ,, A slightly yellow- 
ish - white pre- 
cipitate. 



For the purpose of tanning white and delicately coloured 
leathers, therefore, the collection should be made in June ; while 
for tanning dark-coloured leathers, and for dyeing and calico- 
printing in dark colours, where the slightly yellow shade will 
have no injurious effect, the collection may be made in July. It 
appears that, for all purposes, the sumach collected after the ist 
of August is inferior in quality. 

Experimental results as regards percentage of tannin obtained 
by collecting sumach at different seasons showed : — 







Per cent, of 






Tannic Acid.; 


Virginia, mixed, collected 


in June, gave 


22-75 


,, ,, ,, 


July „ 


. ,27-38 


,, R. glabra ,, 


August ,, 


23-56 


,, R. copallina ,, 


,, ,, 


16-99 


Sicilian, R. coriaria ,, 


y) )> 


24-27 



It is evident, therefore, that in order to secure the maximum 
amount of tannic acid the sumach should be collected in July, 
but the colouring matter of the leaves has an important influence 
upon the value of the product. The leaves of the upper ex- 
tremities of the stalks are always richer in tannic acid than those 



VEGETABLE TANNING MATERIALS 315 



of the base ; and the increase of age of the plant is accompanied 
by a general diminution of this acid. 

The mill used for grinding sumach leaves consists of a heavy, 
solid, circular, wooden bed, 15 feet diameter, with a depression 
around the edge a few inches deep and i foot wide, for the recep- 
tion of the ground sumach from the bed, and two edge-roUers, 
weighing about 2500 lb. each, 
5 to 6 feet diameter, and pro- 
vided with numerous teeth of 
iron or wood, thickly inserted. 
In Europe and in some parts 
of the Southern States sumach 
is still ground by stones re- 
volving on a stone bed, and 
the sifting is often done by 
hand. 

R. cotinus, Venetian sumach. 
Fr. Arhre a perruques ; Ger. 
Pernikenstrauch (fig. 67). In 
Europe it grows in the Apen- 
nines, but is said to be common 
in the West Indian islands. 
More important as a dyeing 
t-han as a tanning material, 
its twigs and wood, " young 
fustic," containing a large pro- 
portion of a colouring matter 
(fisetin), which with tin and 
alumina mordants dyes bright 
yellows ; and much resembles, 

but is not identical with, the myricetin present in R. coriaria.^ 
Its leaves, known as Turkish or Venetian sumach, contain about 
17 per cent, of tannin, and are used for tanning. 

R. pentaphylla, " Tezera," Algeria, is used by the Arabs for 
tanning goat-skins. 

R. Thunhergii, Kliphout, Cape of Good Hope. A sample of 
the bark analysed in the Author's laboratory contained 28 per 
cent, of tanning matter. A valuable tanning material, of reddish 
colour. The tannin is of the catechol class. 

Several other species of Rhus are used in tanning. R. 

semialata yields Chinese and Japanese galls, containing up 

to 70 per cent, gallotannic acid. They are caused, not by a 

fly, but by the attack of an aphis, as are those of the allied 

^ Perkiii and Allen, Trans. Chem. Soc, 1896, 1299. 




Fig. 



67. — Venetian Sumach 
{Rhus cotinus). 



3i6 PRINCIPLES OF LEATHER MANUFACTURE 

Pistacia.i The asphides pass their asexual stage inside the gall, 
which is large and thin-walled. A similar aphis-gall is found on 
the American sumach.. A specimen of the leaves examined at 
Leeds University yielded only 5 per cent, of tannin. 

R. succedanea (L.). India. Leaves said to contain 20 per cent, 
tannin. 

R. Mysorensis. S. India. Bark gives pink colour to chromed 
hide powder, and is a pyrogallol tannin. Analysis (Pilgrim, dry 
material), tannin 19-5 per cent., non-tannin 11-4 per cent, on 
dry material. 

Mangifera indica, Mango, widely distributed in the Tropics. 
Bark and leaves rich in tannin, which gives green-blacks with 
iron. 

CORIARIACE.E 

Coriaria myrtifolia, French sumach (of which there are four 
kinds — fauvis, douzere, redoul or redon, and pudis) . A poisonous 
shrub of South of France ; leaves used for tanning, and as a 
sumach adulterant under the name of " stinco " ; contain about 
15 per cent, tannin [cp. p. 309). 

C. ruscifolia bark, the tutu of New Zealand, contains 16 to 17 
per cent, of tannin. 

C. nepalensis (Wall), Eng. India. Leaves 20 per cent., all parts 
rich in tannin. 

Other Coriarias merit examination, and are known to contain 
much tannin. 

RUBIACE^ 

Riibia, Madder, allied to Galiums, which are almost the only 
English representatives of the family. The coffee- and cinchona- 
plants are foreign representatives. 

Nauclea (or Uncaria) gambir. East Indies (fig. 68). A 
climbing shrub, source of " gambier," or " Terra Japonica " ; 
also called " Catechu," in common with several other solid 
extracts. Gambier is first described by the Dutch trader 
Couperus in 1780 ; plant introduced in Malacca, 1758 ; planta- 
tions established in Singapore in 1819. 

Culture is mainly by Chinamen, and is very rude ; it yields 
rapid return, but under the treatment to which it is subjected a 
plantation is worn out in ten to fifteen years. Cropping com- 
mences three years after planting, and is continued two to four 
times annually with little regard to fitness of shrubs, the plant 
^ See Fliickiger and Hanbury, Pharmacographia. 



VEGETABLE TANNING MATERIALS 317 

being cropped till it has barely leaves left to support existence. 
It is found advantageous to combine pepper-culture with that of 
gambler ; the spent leaves form a good protection for the pepper- 
plant roots, but they have Httle actual manurial value. 

Cropping is done with a knife called a parang, while a larger 




Fig. 68. — Gambler Shrub (Nauclea , 



knife is used for chopping the leaves and twigs before they are 
put in a boiler, in which they are heated with water till the liquid, 
which is constantly stirred during the operation with a wooden 
five-pronged stirrer, becomes syrupy. The leaves are then 
brought out with a wooden fork, and allowed to drain on a tray, 
so that the liquor runs back into the boiler. The coarser matter 
still remaining in the boiler is removed with a strainer like a 
racquet, and the finer by straining the liquor through a perforated 
cocoanut shell into small shallow tubs, where it is allowed to 
cool with constant stirring with a cylindrical wooden bar, which 



3i8 PRINCIPLES OF LEATHER MANUFACTURE 

is worked up and down with a rotary motion until the catechin 
crystalhses. When quite cool the pasty mass is turned out of 
the tub, cut into cubes with sides i inch long with a hoop-iron 
knife, and dried on bamboo trays in racks under sheds, or some- 
times smoke-dried with wood fires. 

Good cube gambler is an earthy-looking substance, and is 
dark outside, but pale within from crystallisation of catechin. 
Catechin is not itself a tanning material, but is apparently con- 
verted into a tannin by drying at iio° to 126° C, when it parts 
with a molecule of water. It is very probable that a similar 
change occurs in the tannery. The tannin is a catechol-phloro- 
glucol derivative, less astringent than most of this series, and of 
pale colour (see p. 339). 

A commoner quality, called " block-gambier," instead of being 
cut into cubes, is run into large oblong blocks of about 250 lb. 
weight, which are wrapped in matting and exported in a pasty 
condition. These contain 35 to 40 per cent, of tannin, as esti- 
mated by the hide-powder method, while the best cubes reach 
50 to 65 per cent. Besides the forms named, various others are 
made, principally for native use in chewing with betel-nut in 
the form of small biscuits, or in thin discs (" wafer gambler ") 
by running the pasty mass into bamboos and cutting the cylinder 
so formed into thin slices. These forms are usually light in 
colour, and very rich in catechin. 

For details of the chemistry of gambier see L.I.L.B., 157. 

Uncaria acida (Roxh.). Moluccas, Java. 

U. Bernaysii (F. v. M.). New Guinea. 

U. dasyoneura (Thwaites). Ceylon. Are said also to yield 
gambier 

APOCYNACE^ 

Carissa spinarum (Karunda). C. and N. India. Bush. 
Leaves used in mixture. Analysis (Brumwell), tannin 8-o to 
11-5 per cent., non-tannins 13-5 to 15-0 per cent. Not very 
easily absorbed by hide. Infusion pale in colour and of extreme 
swelling power. According to Fraymonth and Pilgrim pro- 
duces very tough leather. 

Aspidospermum quebracho. Sp. Quebracho bianco. Brazil. 
Bark contains aspidospermin, an alkaloid used in medicine, but 
both bark and wood are poor in tannin. 

Quebracho Colorado, see Anacardiace^, p. 306. 



VEGETABLE TANNING MATERIALS 319 

ERICACE^, Heath Family 

Arctostaphylos (or Arbutus) uva-ursi, Bearberry. Used in 
Russia, Finland ; twigs and leaves said to contain 14 per cent, 
tannin. Often adulterated with leaves of Vaccinium vitis-idce 
or Cowberry. 

Arbutus unedo, Common Arbutus. Leaves, fruit, and bark 
used on Mediterranean coasts. 

VACCINIA 

Vaccinium Myrtillus, Bilberry. Used in Piedmont. 

SAXIFRAGES 

Weimannia glabra L., " Curtidor " bark. Venezuela. 
W. macrostachys D.C. Reunion. 
W. racemosa, New Zealand Towai or Tawheri bark. 
Thes€ species contain 10 to 13 per cent, of iron-blueing tannin, 
and have been practically used, but are not of much importance. 

TAMARISCINIS 

Most of the members of this group are poor in tannin, but 
several species have galls which are rich. 

Tamarix africana, Egypt, Algeria. Galls containing 26 to 56 
per cent, tannin. The small twigs are collected in Tunis, and 
when dried and ground are imported into Sicily to be used for 
the adulteration of sumach under the name of " Brusca," and 
contain about 9 per cent, of tannin {cp. p. 31,0). 

T. articulata, Morocco. Yields galls produced by aphides, called 
in Arabia Takout, and stated by Vogel to contain 43 per cent, of 
tannin. 

T. gallica. Used in Spain and Italy. 

OXALIDEiE 

Oxalis gigantea, source of churco bark. Chili. A thin, brittle, 
dark red bark, mostly about 2 mm. thick, cork and ross entirely 
absent. The bark is brittle, and the cells thin. It contains 
about 25 per cent, of an easily extracted, dark red tannin, giving 
green-blacks with iron. The bark has been incorrectly attributed 
to Fuchsia macrostemma. {Cp. Von Hohnel, Die Gerberinden, 
p. 125, and this book, p. 324.) 



320 PRINCIPLES OF LEATHER MANUFACTURE 

COMBRETACEiE 

Several families of this genus contain trees rich in tannin, 
but most important are the Myrobalans (often, but incorrectly, 



T T 'i 



V 






\ 



Fig. 69. — Myrobalan Tree (Terminalia Chebula). 

written Myrabolams or Myrabolans), the unripe fruit of various 
species of Indian Terminalia. 

Terminalia Chebula (fig. 69), a tree 40 to 50 feet high, and 
yielding good timber, is the source of all the ordinary varieties, 
which differ only in the district from which they are obtained and 
the state of maturity of the fruit. The nuts contain from 30 to 40 
per cent, of tannin. Of the various sorts, probably those known 
as Bombays are least unripe, while " lean greens " are the most so. 
The unripe fruit is the richest in tannin. " Bombays " have a 
smooth skin in coarse wrinkles, and when cut are porous and 
light coloured. " J's " (Jubbalpores) and " V's " (Vingorlas) 



VEGETABLE TANNING MATERIALS 321 

have finer and shallower wrinkles, and are harder, solider, and 
consequently darker looking, but do not give a darker liquor ; 
while " lean greens " are greener, have less yellow colouring 
matter, and consequently more nearly approach in character 
to sumach, which the tannin in many respects resembles, though 
probably containing more ellagitannic acid in proportion to 
gaUotannic acid than the latter. 

The " nuts " should be bright in colour, not worm-eaten, nor 
" waxy " or soft. If kept in a damp place they rapidly absorb 
moisture and fall into the " waxy " condition, in which they are 
very difficult to grind, sticking to and choking the cutters or 
beaters of the mill. 

Neither the large hard stones nor their kernels contain tannin, 
but the latter have an oil which gives a peculiar odour to leather. 
The tannin exists in large and rather thickly-waUed cells, and is 
not very easily extracted ; the skin is wrinkled, but the un- 
crushed nuts swell up to their original plum-like form when 
placed in water for some time. The bark is almost as rich as 
the fruit, and the tree also 3d elds galls. 

T. Belerica yields Beleric or " Bedda nuts," which are downy, 
rounder and larger than ordinary myrobalans, and contain about 
12 per cent, of tannin, used as adulterant of ground myrobalans. 
A sample of solid extract made from the bark of T. Belerica 
contained 70 per cent, of tannin. 

T. tomentosa has downy nuts, containing about 10 per cent, 
of tannin ; bark stated by de Lof to contain 36 per cent, of tannin. 
A sample of solid extract contained 56 per cent, of tannin. The 
bark contains about 11 per cent. 

There are several other Indian species. 

T. Catappa, " Badamier bark " of Mauritius, contains 12 per 
cent, of tannin. 

T. mauritiana, " Jamrosa bark," said to contain 30 per cent, 
of tannin. 

T. Oliveri, Malay Archipelago, yields " Thann leaves," from 
which an extract is made as a cutch substitute. A sample of the 
extract from Burmah examined recently in the Author's labora- 
tory contained 62 per cent, of tannin. The tannin is a catechol 
derivative, differing from that of Acacia catechu in containing no 
phloroglucol (p. 330). 

A sample of bark from Mandalay contained 31 per cent, of 
tannin, while the leaves from the same tree contained 14 per cent. 

T. arjiina. (Kahua.) C. India. Bark gives 18 to 24 per cent, 
tannin (Pilgrim). Light fawn-coloured leather. Bark suitable 
for harness and sole-leather tannage. Apparently mixed tannin. 

21 



322 PRINCIPLES OF LEATHER MANUFACTURE 

T. hraziliensis (Eichl.). Brazil. Used for tanning, 

T. Buceras (Wright). Antilles. Used for tanning. 

A number of other species of Terminalia are rich in tannin. 

Laguncularia racemosa (Gartn.) {Conocarpus racemosus L.). 
West Indies, Brazil, Jamaica, West Africa, " White mangrove," 
" Paletuvier gris." Very rich in tannin.^ 

Anogeissus latifolia. (Dhawa, Dhaura.) C. and S. India. 
Bark and leaves rich in tannin. Dhawa twig bark shows (Pilgrim) 
13-5 per cent, tannins and 137 per cent, non-tannins ; the mature 
bark 15-5 per cent, tannins and 8-8 per cent, non-tannins. Used 
in mixture. Leaves show (Pilgrim, Brumwell, and others) 10 to 
18 per cent, tannin and 12 to 23 per cent, non-tannin. Immature 
leaves or shoots (Dhawa sumach) yield 20 to 30 per cent, tannin 
and 12 to 16 per cent, non-tannin. Red tips of immature shoots, 
separately collected in S. India, showed 54 per cent, tannins and 
14-5 per cent, non-tannins. Mixture of bark and leaves tans 
quickly, giving pale, greenish leather. Good bleacher like 
European sumach. Used as a dye. Gives little swelling. 

The tree grows like a true mangrove in the water on marshy 
coasts and the borders of large rivers. The bark is often mixed 
with other rhizophore barks in extracting, but would give a very 
superior tanning extract if it could be extracted separately. 

RHIZOPHORACEiE, Mangles or Mangroves 

Rhizophora Mangle, and other allied species, Mangrove or 
Mangle, MangUer, Paletuvier, Cascalote, grows on tropical coasts 
all round the world. The barks vary much in strength, from 15 
up to 40 per cent, in different species (see Ceriops). Leaves, 
used in Havana, are said to contain 22 per cent, tannin. Accord- 
ing to Eitner, the younger plants contain the highest proportion 
of tannin. R. Mangle seems to yield a bark inferior to several 
other species. From their mode of growth most mangrove 
barks contain salt. 

All trees growing in swamp, and of the same character of 
growth as mangrove, are called " Bakau " in the East Indies 
{anglice, mangrove), and various species of Ceriops yield the best 
tanning bark. A tidal mangrove swamp at low water is a tangle 
of arched roots like inverted branches on which the trees are 
supported. 

The catechol -tannin, which is easily extracted, is of deep red 
colour and allied to that of the mimosas. In admixture with 

^ Niereiistein and Webster, Quart. Journ. of Com. Research in Tropics, 
i. p. 70, V. p. 35, and Collegium, 7, 1908, p. 161. 



VEGETABLE TANNING MATERIALS 323 

other materials the red colour has a much smaller effect ; the man- 
grove bark is now largely used in combination with pine, oak, 
and mimosa, and also with the synthetic tannins. The tannin 
does not diffuse appreciably through parchment paper, and salt 
can be removed in this way, but does not seem to do much injury 
in tanning. 

Several other species are also rich in tannin, and used in 
different parts of the world under the name of mangle, as are 
also several species of Conocarpus belonging to the Combretacece. 

Rhizophora mucronata. India and Burmah. Bark varies 
considerably ; David Hooper, Indian Museum, Calcutta, gives 
26-9 per cent, of tannin. Dr Koerner (Deutschen Gerberschule, 
Freiberg) analysed two samples in igoo, one containing 48 per' 
cent, and the other 21 per cent, of tannin ; two samples from the 
British Imperial Institute recently examined by the Author 
showed only 4-5 and 6-i per cent, of tannin respectively. 

Ceriops Candolleana, Bakau or Tengah Bark, East Indies. 
Goran, Bengal, South America. Contains up to 40 per cent, of 
tannin, and yields an extract which promises well as a substitute 
for cutch, to which, for dyeing purposes, it is nearly or quite 
equal. The solid extract contains up to 65 per cent, tannin, 
making a good but dark red leather. 

Ceriops Roxburghiana, a somewhat larger tree, also growing 
in the Sunderbans ; bark very similar in strength and character 
to the above. 

Brtigniera Rhumphii, the commonest mangrove on the coasts 
of New Caledonia,^ has a hard red wood, little subject to decay. 
The leaves are oval, and 15 to 20 cm. long, and the fruits are 
shaped like cigars, but 35 to 40 cm. long, and when they drop 
fall point downwards into the mud, where they germinate. (The 
fruits of some mangroves germinate on the trees before they fall.) 
Thuau found 42-6 per cent, of tannin in the trunk bark, but only 
12 per cent, in that of the aerial roots. 

Crossostylis multiflora, " petit paletuvier," ibid., New Cale- 
donia, is a bush -mangrove, which often forms impenetrable 
thickets on the coasts and river banks. Its bark and twigs 
contain but little tannin, but the larger wood reaches 21 per 
cent. 

^ U. J. Thuau, " Tanins de la Foret Caledonienne," Collegium, 7, iyu8, 
P- 376. 



324 PRINCIPLES OF LEATHER MANUFACTURE 



LECYTHIDACE^ 

Couratari domestica or estrellensis. Brazil. 

C. legalis. Brazil, " Jiguitiba rose ; " large forest tree. Bark 
contains tannin. 

C. guianensis. Guiana. Bark " de mahot couratori ; " used 
for tanning. 

Lecythis pisonis (Cambes). Brazil, " sapucara " ; used by 
natives. 

Many other species of Rhizophora, Ceriops, and Bruguiera 
yield barks containing good percentages of tannin. 

ONAGRACE^, the CEnothera Family 

Fuchsia excorticata, the only deciduous tree of New Zealand. 
Contains 5 per cent, tannin. 

Fuchsia macrostemma. Chili. Yields Tilco or Chilco bark. 
Churco bark has been incorrectly attributed to this plant, but it 
is certainly derived from an oxalis, as stated by the Kew authori- 
ties. (Cp. Von Hohnel, Die Gerherinde, p. 125.) 

Ludwigia caparossa, " Caparossa," Brazil. Bark 20 to 25 per 
cent, tannin. Abundant in Minas-Geraes and Goyaz. 

GUNNERACE.E 

Gunner a scabra {Pangue ?), Pauke, Chili. Used occasionally 
in the tanning of goat-skins. 

MYRTACE^ 

Eucalyptus globulus, and other species of Eucalyptus, common 
in .Australia, and introduced into Algeria and Southern Europe 
(gum-trees), are more or less rich in catechol-tannins, their sap 
being the source of Botany Bay or Australian kinos, which contain 
up to 79 per cent, tannin. Several species of Eucalyptus afford 
astringent extracts; those from the " red," " white," or " flooded " 
gum {E. rostrata), the " blood-wood " {E. corymbosa), and E. 
citriodora, being quite suitable for replacing the officinal kind. 
The gum is chiefly obtained by woodcutters, being found in a 
viscid state in flattened cavities in the wood, and soon becoming 
inspissated, hard and brittle. Minor quantities are procured by 
incising the bark of living trees ; a treacly fluid yielding 35 per 
cent, of solid kino on evaporation is thus obtained. The gum 



VEGETABLE TANNING MATERIALS 325 

is imported from Australia, but there are no statistics to show 
in what quantity.^ 

Eucalyptus longifolia bark, the " woolly-butt " of Australia ; 
contains 8-3 per cent, of tannic acid, and 2-8 of gallic. The 
' peppermint " tree contains 20 per cent, of tannic acid in its 
bark. The " stringy-bark " {E. obliqua) gives 13I per cent, of 
kinotannic acid. The Victorian '" iron-bark " {E. leucoxylon) 
contains 22 per cent, of kinotannic acid, but is available only 
for inferior leather. 

E. Occidentalis, " Mallet," " Flat-topped Yate," jdelds the 
valuable mallet bark, containing up to 44 per cent, of a yellow- 
brown tannin free from the red of the mimosas. Tannin 39-9 
per cent., non-tannin 8-i per cent. (PoUak, Coll., 17, 1918, p. 3). 
It is most abundant in Western Australia. Used alone it gives 
a leather somewhat hard and inclined to brittleness, but is quite 
satisfactory in mixture with milder materials. 

Myrtus communis, and several other myrtle species, contain 
a considerable amount of tannin in the bark and leaves. 

Eitgenia hraziliensis (Lam.). Paraguay. Bark stated to con- 
tain 43-4" per cent, tannin; leaves, dry, i6-6 per cent., wood, 
II-6 per cent. (Stockberger, 1912). 

E. or Sizygmm jamjolana. India, and cultivated in Antilles. 
Fruit " jamoon " or " Java plum." Bark 19 per cent, tannin 
(Hooper). 

E. michellii (Lam.). Paraguay. Bark 28-5 per cent, tannin 
(Stockberger). 

Stermolepsis gummifera. New Caledonia. Yields resinous 
matter with 80 per cent, tannin according to Heckel and Schlag- 
denhauffen ; Thuau (1908) found 427 per cent, in resin, and 
17-4 per cent, in bark. 

SAPINDACE^ 

Mostly climbing shrubs, usually rich in saponin, and often 
containing tannin. 

Cupania uraguensis. Paraguay. Bark 17-5 per cent, tannin 
(Stockberger, 1912). 

C. spec. Paraguay, " cedrillo." 15-8 per cent, tannin (Stock- 
berger). 

Paullinia sorhilis or cupana. Brazil and Columbia ; fruit 

" Guara," possibly identical with " cascalote," is very rich in 

tannin ; said to contain up to 55 per cent. T. CaUan found 

43 to 48 per cent, tannin, 23 per cent, non-tannins. (See 

^ Compare Journ. Soc. Chem. Ind., 1902, p. 159. 



326 PRINCIPLES OF LEATHER MANUFACTURE 

J.A.L.C.A., 1915, pp. 241, 646.) Appears to be a mellow 
pyrogallol-tannin ; develops acidity in liquors, and gives crimson 
colour with concentrated H2SO4. 

P. muUiflora. Paraguay. Is also rich in tannin. 

RHAMNACE^ 

Zizyphus Xylopyra. (Gothar, Ghout). C. India. Fruit, in 
appearance like a small plum, contains a hard stone. Flesh and 
stone about equal in weight. Tannin contained in fruit, which 
also yield much glutinous or starchy matter, rendering filtration 
difficult. Said to give cracky leather if used in large proportion. 
Penetrates quickly. Liquor ferments rapidly and then acts as 
a bate. Very abundant and cheap. Analyses (Brumwell) of 
flesh, 23 per cent, tannin and 41-5 per cent, non-tannins. 

Ceanothus velutinas. Colorado, California, " Snow-Bush," 
Leaves contain 17 per cent, of a catechol-tannin, and 7 per cent, 
of a brittle wax soluble in petroleum spirit {J.A.L.C.A., 1916, 
P- 319)- 

GRANATACE^ (PUNICACEiE) 

Punica Granatum. Pomegranate. Rind of fruit used in 
Spain and the East. Substitute for sumach. Analyses (Brum- 
well) show 27 to 30 per cent, tannin and 18 to 20 per cent, non- 
tannin on dry material. Bark of tree said to contain 22 per cent, 
of tannin. Balaustines, wild pomegranates, E. Indies, up to 46 
per cent, tannin in fruit. 

ROSACEiE 

Tormentilla erecta, Potentilla tormentilla. Root variously 
stated to contain 20 to 46 per cent, tannin. Red-coloured leather, 
formerly used in Orkneys, Shetland, and Faroe Islands, and in 
some parts of Germany. 

Sorbus or Pyrus Aucuparia, Mountain Ash. Bark said to be 
stronger than oak. 

Many other plants of the family contain tannin, among others 
the strawberry. 

PAPILIONACE^ 

Biitea frondosa.^ This (with Pterocarpus marsupium) ^ fur- 
nishes East Indian kino. The flowers are used in India as a 

^ Dictionary of Economic Products, I.B., p. 944 ; Hummel and Cavallo, 
Proc. Chem. Soc, 1894, p. ii. 

2 Agricult. Ledger, 1901, No. 11, Government Printing Office, Calcutta. 



VEGETABLE TANNING MATERIALS 327 

yellow dye under the name of Tesu. Bark fairly rich in 
tannin. 

Pierocarpus or Drepanocarpus senegalensis is the source of 
African kino, which contains up to 75 per cent, of tannin. 

CcBsalpinia coriaria, Divi-divi. A tree of 20 to 30 feet, native 
in Central America, introduced successfully in India, but princi- 
pally imported from Maracaibo, Paraiba, and Rio Hache. In 
Curagoa it is a large tree up to 21 inches diameter, with dark 
and very hard and heavy timber. In Venezuela the local name 
is " nacasae " or " wouta pana." The dried pods contain 40 to 
45 per cent, of a pyrogallol-tannin, mainly ellagitannic acid, 
and would be a most valuable tanning material but for a liability 
to fermentation and sudden development of a deep red colouring 
matter. The causes are not well understood, but apparently 
the risk can be materially lessened by use of antiseptics, and the 
writer, who has used a good deal, has never seen a case. If used 
in strong liquors it gives a heavy and firm leather, but is princi- 
pally employed as a partial substitute for gambler on dressing 
leather. Used in rapid drum-tannage for light leathers an 
excellent colour may be obtained. It is said to give an especially 
firm and glossy flesh. Leather tanned with it, even when of 
outwardly good colour, has often a blueish-violet shade within, 
perhaps due to the development of a colouring matter allied to 
that of logwood. The seeds do not contain tannin, which lies 
almost free in the husk of the pod. The pods are about 3 to 4 
cm. long, dark outside, and curl up in drying to an S-shape. 

If passed through a disintegrator to break the pods a very 
rich tanning powder can be sifted out, mostly soluble in water. 

C. digyna, tari or teri pods. Occurs in Prome, Toungoo, 
Bassin, Mynang, and other parts of India and Burmah, where it 
is used as a drug. The pod-case is said to yield over 50 per 
cent, of tanning matter. A sample from Burmah, kindly sent 
by the Imperial Institute, examined by the Author in 1900, 
contained 24 per cent, of tannin, but after removing the seeds 
the remaining pod-cases yielded 44 per cent, of tannin on analysis. 
C. digyna promises to become a valuable tanning material if it 
proves free from the tendency to ferment which is so trouble- 
some in divi-divi. It has been introduced into England under 
the name of " white tan," which yields a leather quite as white 
as sumach ; but the supply seems at present uncertain. 

C. cacolaco, Cascalote, Mexico. Pods rich in tannin (up to 
55 per cent., Eitner). Pods larger and fleshier than divi, seeds 
smaller, tannin similar. 

The pods of several other GcBsalpinias are used in tanning, 



328 PRINCIPLES OF LEATHER MANUFACTURE 

sometimes under the name of " Algarobilla," which is simply a 
diminutive of Algaroba, the carob, or locust-bean, derived from 





Fig. 70. — Algarobilla [Ccesalpinia brevifolia). 

Arabic al Kharroba, and applied to several small pods. (See 
Balsamocarpon and Prosopis.) 

C. (or Balsamocarpon) brevifolia, Chili, ordinary Algarobilla 
(fig. 70) .1 One of the strongest tanning materials known, con- 
taining an average of 45 per cent, of a tannin very like that of 
divi, but less prone to discoloration. The tannin lies loose in a 
^ New Commercial Drugs and Plants, No. 5, T. Christy. 



VEGETABLE TANNING MATERIALS 329 

very open skeleton of fibre, and is easily soluble in cold water ; 
the seeds contain no tannin. If not allowed to ferment it pro- 
duces a very bright-coloured leather. 

Algarobilla has been attributed to Prosopis pallida, but this 
appears incorrect. Several species of Prosopis are known to yield 
tanning pods ; those of P. Stephaniana of the desert of Kaschan, 
in Persia, are dschigh dschighe, perhaps identical with dchift or 
jaft. (See p. 298.) Bark of P. spicigera used in Punjab. 

C. (or HcBmatoxylon) campechianum, Logwood. Central America. 
In addition to colouring matter, and a glucoside which it yields 
on oxidation, this wood contains about 3 per cent, tannin. Its 
principal use is in dyeing blacks with iron or chrome mordants. 
(See p. 493.) Trees (" Bastard Logwood ") occur, apparently of 
the same species, but devoid of colouring matter. 

C. tinctoria (?), " Cevelina " or " Celavina." Central America 
and western South America. Pod verj^ rich in almost white 
pyrogallol-tannin ; gives no bloom. 

C. echinata jdelds " Brazil-wood." (See p. 505.) 
C. Sappan, Sappan-wood, India. 

Cassia auriculata, Turwad, Avar am, or Tanghadi bark. Southern 
India. Used for tanning so-called " Persian " sheep and goat- 
skins ; contains about 17 per cent, of a catechol-tannin. Leather 
tanned with it is of a paJe yeUow colour, but rapidly reddens in 
sunlight {cp. p. 498). The bark is small and thin, and curls up 
like cinnamon, hence its name cassia. It is one of the most 
important materials of India, but somewhat costly as compared 
to babul. 

Cassia fistula (Amaltas, Sonari, Konnai). S. India. Bark 
contains up to 14-5 per cent, tannin, i6-o per cent, non-tannins 
(Brumwell). Gives smooth -grained almost white leather. 
Usually employed in mixture with turwad or babul. Husk of 
pod contains 17 per cent, of tannin. Pulp of pod used as an 
aperient. 

C. elongata and lanceolata. Senna leaves. Upper Egypt. 

C. Sophora,. " Bali-habilan." 

Peltophorium dubiuni. Paraguay. Bark 31-2 per cent, tannin. 

MIMOSE^, a Tribe of Leguminosae 

Acacia arabica, " Babool," " Babul," India, " garad " or 
" Sunt " pods, Sudan, " Gambler pods " (fig. 71). Bark contains 
about 12 to 20 per cent, of catechol-tannin ; one of the principal 
Indian tanning materials, used for kips and heavier leathers. 
Pods, used in India for bating, contain about same amount of 



330 PRINCIPLES OF LEATHER MANUFACTURE 

tannin as bark, but of a different kind, that of the bark being a 
catechol-tannin, with a good deal of red colouring matter, while 
the pods contain a paler tannin allied to divi, which is not preci- 
pitated by lime-water. In Egypt the pods are called hahlah, a 
name which is also applied to pods of A. cineraria and A. vera 
and others. They are used for dyeing glove-leathers. 



iJ.4€*t^ 




# 


15 




»> 


'^^ 


• 'm^ 






i 

.1 5 




\ 

1 

• , 1 






v/ 



Fig. 71. — Babool {Acacia arahica). 



A. nilitica, Egypt. Pods called neb-neb or bablah. 

A. catechu, India. The wood yields cutch or " dark catechu." 
A lighter coloured variety called kath, containing much crystal- 
lised catechin, is also made in India, and principally used for 
chewing with betel. A. catechu is a tree 30 to 40 feet high, com- 
mon in India and Burmah, and also in tropical East Africa, where, 
however, it is not utilised. In Southern India, A. sttma is also 
used for the same purpose. 

Trees of about i foot diameter are cut down, and the wood 
(some state the heart -wood only) is reduced to chips, and boiled 



VEGETABLE TANNING MATERIALS 331 

with water in earthen jars over a mud-fireplace. As the hquor 
becomes thick and strong it is decanted into another vessel, and 
the evaporation continued till the extract will set on cooling, 
when it is poured into moulds made of leaves or clay, the drying 
being completed by exposure to the sun and air. " Kath," or pale 




Fig. 



-Cutch Tree (A cacia catechu) . 



cutch, is made in Northern India by stopping the evaporation at 
an earlier point, and allowing the liquor to cool and crystallise 
over twigs and leaves thrown into pots for the purpose. It con- 
tains a large proportion of catechin, apparently identical with 
that of gambler, but its tannin is much redder. Good cutch 
contains about 60 per cent, tanning matter, but is principally 
used for dyeing browns and blacks with chrome and iron mordants. 
It contains quercetin, a yellow colouring matter (p. 298). 

A. leucophlea, India and Java " Pilang." Pods and bark equal 
to A. arabica. 



332 PRINCIPLES OF LEATHER MANUFACTURE 

Australia abounds in acacias (mimosas), many of which are 
used in tanning, but vary greatly in strength, not only according 
to species, but probably also by situation and growth. Probably 
the best information is to be found in a pamphlet on Wattles 
and Wattle-Bark, by J. H. Maiden, F.L.S., published by the 
Department of Public Instruction at Sydney, 1890. His analyses 
were made by the Lowenthal process, and can only be roughly 
compared with those by the hide-powder method. The analyses 
given are by the I.A.L.T.C. method, and mostly on samples 
furnished by Mr Maiden. 

A peculiarity largely developed in the mimosa family is the 
tendency for the true leaves to be suppressed, and their place 
taken by the flattened and expanded midrib (phyllode). Thus 
leaves of two very distinct forms are common in the genus, and 
some acacias, as A. heterophylla, may have both forms on the 
same branch. Compare A. pycnantha and A. dectcrrens. 

The Australian mimosas have been naturalised in India, and 
grow freely in the Nilgiri Hills, but the bark does not appear to 
be utilised. 

The most important species are as foUows : — 

A. pycnantha (fig. 73). " Broad-leaved " or " Golden Wattle," 
South Australia. One of the strongest tanning barks known. 
A sample marked " special," analysed in the Yorkshire College, 
contained 50 per cent, of tannin ; another sample marked 
" ordinary " contained 40 per cent. 

A. longifolia, the Golden Wattle of New South Wales, only 
contains half as much tannin as A. pycnantha. 

A. mollissima, with its two varieties A. decurrens (fig. 74) and 
A. dealhata, are among the most important of the Wattle family 
commercially. Two samples of the former marked " Green 
Wattle " showed 36 to 39 per cent, of tanning matter ; another 
sample marked " Sydney Green Wattle " contained 41 per cent. 
A sample of A. decurrens, the second variety, was much weaker, 
showing only 12 per cent, on analysis. 

A . penninervis (Hickory bark) is said to be particularly hardy, 
but its strength seems to vary. A sample from Bateman's 
Bay contained 38 per cent, of tanning matter. 

A. hinervata, another " Black Wattle," contains up to 30 per 
cent, tanning matter, as does also the " Weeping Willow," A. 
saligna. The latter is poisonous, and is said to be used for 
killing fish. 

A. prominens, the bark of which resembles that of the Golden 
Wattle, A. longifolia, in appearance contains only 14 per cent, 
tannin. 



VEGETABLE TANNING MATERIALS 333 

The cultivation of wattles in Australia has been somewhat 
neglected, but would render possible the utilisation of many 
acres of land lying waste, or which have already been exhausted 
and rendered unfit for the growth of cereals. It requires so 
httle attention as to make it very profitable, and wattle-growing 
and sheep-grazing can be combined satisfactorily after the first 
year, when the young trees in the plantation have reached the 




Fig. 73. — Broad-leaved Wattle 
{Acacia pycnantha). 



Fig. 74. — Green Wattle 
{A cacia decurrens) . 



height of 3 to 4 feet. In Natal the Australian wattles, especially 
A. molUssima, have been acclimatised and cultivated with 
success, and large quantities of excellent bark are now exported 
to England. African wattle-barks usually contain about 30 per 
cent, of tannin. 

Wattles grow in almost any soil, even the poorest, but their 
growth is most rapid on loose, sandy patches, or where the sur- 
face has been broken for agricultural purposes. When the soil 
is hard and firm, plough-furrows should be made at a regular 
distance of 6 to 8 feet apart, and the seeds dropped into these. 
The seed should be sown in May, having been previously soaked 
in hot water, a little below boiling temperature, in which they 
may be allowed to remain for a few hours. It should be dropped 
at an average distance of i foot apart along the furrow, in which 
case about 7200 seeds would suffice for one acre of land. The 



334 PRINCIPLES OF LEATHER MANUFACTURE 

seed should not be covered with more than about | mch of 
soil. 

On loose sandy soil it might even be unnecessary to break 
up the ground in any way ; the furrows may be dispensed with, 
and the seed sown broadcast after the land has been harrowed. 
After the plants have come up, they should be thinned so that 
they stand 6 to 8 feet apart. When the young trees have attained 
the height of 3 to 4 feet the lower branches should be pruned off, 
and every effort afterwards made to keep the stem straight and 
clear, in order to facilitate the stripping, and induce an increased 
yield of bark. It is advisable that the black and broad-leaved 
kinds should be grown separately, as the black wattle, being 
of much larger and quicker growth, would oppress the slower- 
growing broad-leaved one. Care should be taken to replace 
every tree stripped by re-sowing, in order that there should be 
as little variation in the yield as possible. In Victoria, the 
months of September to December are those in which the sap 
rises without intermission and the bark is charged with tannin. 
Analysis proves that the bark from trees growing on limestone 
is greatly inferior in tannin to that obtained from other forma- 
tions, differing 10 to 25 per cent. 

The following are South American mimosas : — 

A. cavenia, Espinillo. Bark contains 6 per cent., pods 18 to 
21 per cent, or more of tannin. 

A. cebil, Red Cebil. Bark contains 10 to 15 per cent., leaves 
6 to 7 per cent, tannin. Argentine Republic. 

A. Guarensis, AlgarobiUa of Argentine Republic. Bark, pods, 
and flowers said to be used for tanning. 

A. timho or Piptadenia. Buenos Ayres. 

A. cuntpi, Curupy bark. Wichmann found 18-3 per cent, 
tannin. 

A. angico, or Piptadenia macrocarpa (or rigida), Brazil, yields 
" angica bark," a sample of which contained 20 per cent, of 
tanning matter when analysed in the Author's laboratory. Said 
to contain 20 to 25 per cent. 

" White Bark," South America, probably an acacia ; bark 
internally very similar to angica, if not identical. 

A. horrida, " Doornbosch," Cape of Good Hope, bark contains 
8 per cent, of tannin. 

The following are other leguminous plants containing tannin : — 

Inga affinis, D.C. Bark 25-8 per cent, tannin. Paraguay. 

Inga feiiillei, " Paypay," Peru. Pods said to contain 12 to 15 
per cent, of tannin (doubtful). Several other species of Inga 
known to contain tannin. 



VEGETABLE TANNING MATERIALS 335 

Elephantorrhiza Burchellii, Elandsboschjes, Tugwar, or Tul- 
wah, South Africa ; a papilionaceous plant. The air-dry root 
contains 12 per cent, of tannin and a great deal of red colouring 
matter. The roots are several feet long and about 2 inches in 
diameter, growing by the sides of rivers. 

Acacia or Alhizzia gramilosa. New Caledonia. Fine timber 
tree like walnut ; bark contains 12 per cent, tannin. 

Stryphnodendfon harhatimao, Brazil, " barbatimao " bark. 
Paessler found 18 to 27 per cent, tannin. 

Pithecolohium duke. Mexico, Philippines, Malay Archipelago. 
Bark said to contain 25 per cent, tannin. Mexican name " caman- 
chile." Other Pithecolobia contain tannin. 

P. gummifera, Brazil, " Arnhatico di Campo." 

Xylia dolabriformis (Benth.). An extract has been made from 
the wood sawdust. The yield was small, but the dry extract is 
said to have contained 85 per cent, tannin. 

Enter olohium timbouva (Mart.), " timbo " bark. Paraguay. 
Bark 22-3 per cent, tannin. 

E. monjolo, Brazil, " Monjolo branco de Espinho." Used 
extensively. 

Bauhinia vahlii. (Mahurain.) Creeper growing in C. India. 
Cementing matter between fibres contains tannin and can be 
" combed " out. Fibre useful for rope-making. Tannin gives 
cream-coloured smooth-grained leather, but does not penetrate 
rapidly. Liquors have good swelling power. 

GUTTIFER^ 

Garcinia mangostana. India. The rind of the mangosteen 
fruit contains much tannin. 

Rheedia Braziliensis, Paraguay, " Pakuri." Bark 21-6 per 
cent, tannin and little colour. 

DIPTEROCARPACE^ 

Shorea robusta. (Sal-bark.) India. Analyses by Brurnwell 
show 6-2 to 15 per cent, tannin, which is found in the dust after 
disintegration ; fibre remaining contains not more than 3 per 
cent, tannin. It is a valuable material, giving a pale, tough 
leather, when used in mixture with gothar and karunda. 

Valeria Indica (L.). India. Fruit contains 25 per cent, of a 
pale tannin. 

Dipterocarpus tuberculata. India. An extract has been made, 
but much of the tannin became insoluble in the process. 



336 PRINCIPLES OF LEATHER MANUFACTURE 



LYTHRACE^ 

Woodfordia florihunda. (Itcha, Thawai). India. Bark and 
leaves contain tannin. Former used for sole leather ; gives good 
colour ; but cracky grain when used alone. Leaves contain 15-4 
per cent, tannin and 17-2 per cent, non-tannin ; bark, 26-6 per 
cent, and 13 per cent. (Brumwell) . 

Thefollowing the writer has not been able to place botanically: — 
Eucoupia cordifolia, Santiago, Ulmo bark. A considerable 
quantity was imported into Germany, probably for extract- 
making. 



CHAPTER XIX 

THE CHEMISTRY OF THE TANNINS 

The essential constituents of tanning materials are members of 
a large group of organic compounds known as " tannins " or 
" tannic acids," which are widely distributed throughout the 
vegetable kingdom, and said to have at least one representative 
among animals, in the body of the -corn- weevil. Their use in 
vegetable physiology is as yet uncertain, and indeed they appear 
in some cases to be waste products of organic change. The 
tannins, though varying considerably in their chemical consti- 
tution, are all marked by the power of precipitating gelatin and 
other proteins from solution, of converting animal skin into 
imputrescible leather, and of forming dark-coloured compounds 
with ferric salts, which are often used as inks. They are also 
precipitated by lead and copper acetates, stannous chloride, etc., 
and form insoluble compounds with many organic bases, particu- 
larly the alkaloids such as quinine and strychnine, and with basic 
aniline colours. They have a feebly acid character. Finally 
what is perhaps the most important fact must be mentioned, 
namely, that they are in aqueous solution negatively charged 
colloids. 

With regard to less characteristic properties it may be said 
that tannins are all soluble in water, but like other colloids have 
no definite solubihties. They are also soluble, in part at least, 
in alcohol, ethyl and amyl acetates, aqueous ether, acetone, etc., 
but not soluble in the fat solvents, including dry ether. Since 
tannins are uncrystallisable and without definite melting points, 
their preparation in a state of purity is a matter of the greatest 
difficulty. Indeed gallotannin is perhaps the only member of 
the class that has been prepared pure. Methods used for other 
tannins are very empirical and tedious, and their description is 
outside the scope of a general account. 

A classification of tannins according to chemical constitution 
is difficult, since so little is known of them in this respect. Perhaps 
the most useful classification is the early one based on the colour 
reaction with ferric salts, i.e. the division into the two classes : 
pyrogallol (iron-blueing) and catechol (iron-greening) tannins. It 

337 22 



338 PRINCIPLES OF LEATHER MANUFACTURE 

has been found that tannins giving a blue colour with ferric salts 
jdeld pyrogallol on dry distillation or sodium gaUate on fusion 
with alkali, whilst those giving a green colour yield catechol or 
the sodium salt of protocatechuic acid. These bodies are phenols 
or phenolic acids. The phenols are a class of derivatives of 
benzene CgHg, in which one or more of the hydrogen atoms are 
replaced by hydroxyl ( — OH) groups. Common phenol or car- 
bolic acid is the simplest representative : 

F^ OH OH OH 



CH CH /^. ,/\, i-^^iOH HO,/ \OH 
I II or 

• CH CH ' J I I I I 

\ / ^ V/ \/ 

CH Phenol. Catechol. Pyrogallol. 
Benzene. 

In the phenolic acids a further hydrogen atom in the ring or 

nucleus is replaced by the carboxyl (— COOH) group, and thus 
a true acid is formed : 



OH 

,/\cOOH 


OH 
/^OH 


HO 
HO^OH 


1 1 
Salicylic acid. 


COOH 
Protocatechuic acid. 


COOH 
Gallic acid. 



How these bodies are linked together in the tannin molecule is 
not known with certainty except in the case of gallotannin, nor 
how they are combined with glucose, as often appears to be the 
case. A. G. Perkin suggests the subdivision of the p3ArogaUol 
class into two others, namely, gallotannins and ellagitannins, or, 
better, into " depside " tannins and ellagitannins. The last- 
named class contains an ellagic acid nucleus, and yields this acid 
(" bloom ") on fermentation, or hydrolysis with mineral acid. 
This treatment with gallotannins gives rise to gallic acid. From 
the point of view of leather technology, however, this further 
classification is of little moment, as it almost amounts to pro- 
viding a separate class for gallotannin. 

The two classes, pyrogallol and catechol tannins, are fairly 
clearly distinguishable by other reactions. Boihng with dilute 
sulphuric acid gives a precipitate of " reds " or phlobaphenes 
with catechol tannins. The behaviour of the pyrogallol group 
has been mentioned above. Bromine water added till the solu- 
tion smells strongly of it gives a yellow or brown, occasionally 



THE CHEMISTRY OF THE TANNINS 339 

crystalline, precipitate with catechol tannins only. Another 
reaction which is generally characteristic of catechol tannins is 
that if concentrated sulphuric acid is added to a single drop of 
infusion in a test-tube a dark red or crimson ring is formed at 
the junction of the two fluids, and on dilution with water the 
colour is generally pink. Pyrogallol tannins, on the other hand, 
give a yellow or brown ring, which dilutes to a yellow solution. 
This reaction is of great delicacy. The formaldehyde test, due 
to Stiasny, gives another means of distinction. Catechol tannins 
when boiled with formaldehyde and hydrochloric acid are com- 
pletely precipitated, pyrogallol tannins incompletely or not at all. 
The precipitate obtained by adding lead acetate to a tannin 
solution is soluble in acetic acid in the case of catechol tannins, 
partially at most in the case of pyrogallol tannins. Finally it 
may be mentioned that the catechol tannins contain as a rule 
about 60 per cent, of carbon, those of the pyrogallol group only 
about 52 per cent. 

The differences mentioned above hold generally, but not 
without exception. An important case is that of oak-bark, 
which contains both types of tannin. This may be the 
reason why no material can be so satisfactorily used alone as 
oak-bark. 

The commoner tanning materials fall into the two classes 
described as follows : — 

Pyrogallol tans : Gallotannin, sumach, oakwood, chestnut, 
myrobalans, valonia, divi-divi, algarobilla. 

Catechol tans : Pine barks (including hemlock), larch, acacias 
and mimosas, quebracho, mangrove, canaigre, and gambler. 

The tannin of oak-bark appears to be principally catechol 
tannin. 

In the case of only one tannin, gallotannin, can it be said that 
the chemical constitution is known. Other tannins have been 
investigated, but without much success ; certain chemical groups 
have been recognised in them and so forth, but little funda- 
mental knowledge has been gained. The case of gallotannin, 
however, is interesting and merits some remark, since it has been 
worked upon since the time of Liebig, but has only very recently 
(1919) been finally settled. The empirical composition of sup- 
posedly pure specimens agreed' closely with the formula Ci^H^oOg, 
and on hydrolysis the principal product was gallic acid. Many 
workers, however, found glucose on hydrolysis, and much dis- 
cussion centred on the question as to whether this glucose was 
part of the tannin molecule or simply an associated impurity. 
Gradually the latter view prevailed, and for many years the view 



340 PRINCIPLES OF LEATHER MANUFACTURE 

of Schiff was adopted, namely, that gallotannin was w-digallic 
acid: 

HO OH OH 

ho/ Ncoo/ y 

HO COOH 

There were difficulties, however, in this view, even though practi- 
cally all of the reactions of tannin could be explained by it. In 
the first place, the molecular weight determinations carried out 
on tannin, though in very poor agreement, were without excep- 
tion very much higher than that of a digallic acid. Secondly, 
tannin is always optically active ; there should be therefore at 
least one asymmetric carbon atom in the molecule, i.e. a carbon 
atom with each of its four bonds attached to different atoms or 
groups. This is not provided for in Schiff's formula. Another 
difficulty was that glucose was summarily ruled out as a con- 
stituent of the molecule. The subsequent work of Fischer and 
his collaborators has led to a much more satisfactory formula. 
At the outset they adopted a method of purification previously 
used b)y Perkin and Stiasny which implied a disbelief in the 
presence of a carboxyl group. This method was to neutralise 
a tannin solution to litmus with dilute alkali and extract the 
tannin with a solvent. This was to leave behind gallic acid 
as sodium salt, but would obviously do the same with any 
digallic acid present. Phenols, other than phenolic acids, would 
not be neutralised under the circumstances. Tannin thus purified 
was found to yield 7 to 8 per cent, of glucose on hydrolysis. 
The details of Fischer's subsequent work are too complex for 
description here. He had, however, before 1913, brought 
forward the strongest evidence that gallotannin was an ester of 
one molecule of glucose and five molecules of digallic acid, i.e. 
penta-digalloyl-glucose : 

9^2*-^^ OH HO OH 

(CHOR)4 where R=-OOc/ \oH <f \0H 
CHO ~~0 0C~ 

The synthesis of this body in the laboratory was hindered by 
many difficulties, but finally Fischer prepared a substance 
identical with the tannin of Chinese galls save in one respect, 
the specific optical rotation in aqueous solution. In organic 
solvents the optical activity presents no difficulty. There can be 
no reasonable doubt that Fischer has produced at least an 
isomer of natural tannin. Dr Freudenberg, who was associated 



THE CHEMISTRY OF THE TANNINS 341 

with Fischer in much of this work, is continuing investigations 
on tannins, particularly chebulinic acid from myrobalans and 
hamameli tannin from Hamamelis virginiana. This latter body 
is exceptional in being crystalhne. 

It has been mentioned above that solutions of pyrogallol 
tannins deposit " bloom " on standing. This is particularly the 
case with valonia, divi-divi, and myrobalans. A similar preci- 
pitate is obtained on boiling with dilute acid, especially in the 
presence of an oxidising agent such as hydrogen peroxide. 
" Bloom " is ellagic acid, and has been shown to have the con- 
stitution 

_ /co-o OH 

HO<__\ /__>0H 

HO \o_oc/ 

Possibly the ellagic acid is formed as follows : hydrolysis of the 
tannin, either by fermentation or the action of acid, results in the 
formation of gallic acid ; two molecules of gallic acid then unite, 
partly by oxidation and partly by loss of water, to form ellagic 
acid: 

co'oH HO on .CO— O. Qj^ 

HO OiH HOOC HO \o_OC/ 

To understand the above it must be remembered that at the 
corners of the hexagon representing benzene a =CH group is 
always present except when a substituting group has been intro- 
duced ; such a group replaces the H of the =CH group. Two 
hydrogen atoms have been written in the left-hand side of the 
equation to indicate where the oxidation takes place. 

Numerous methods have been devised for the synthesis of 
ellagic acid, but they can all be represented by the equation given, 
since they only differ with regard to the particular oxidising agent 
used. Lowe oxidised gallic acid with arsenic acid, Ernst and 
Zwenger boiled ethyl gallate with sodium carbonate solution in 
presence of air, Griessmayer oxidised with water and iodine ; 
the best methods are those of Perkin, in which potassium per- 
sulphate and acid are used, and of Rupe, in which acid and sodium 
nitrite are used. 

Prepared synthetically or otherwise, ellagic acid is a more or 
less yellow, sandy, or crystalline substance of remarkable stability. 
It does not melt below 360° C, but sublimes with considerable 
decomposition at higher temperatures. Its almost complete in- 



342 PRINCIPLES OF LEATHER MANUFACTURE 

solubility in the usual organic solvents was a great obstacle to its 
purification until Perkin found that it dissolved easily in pyridine, 
from which it could be crystaUised in prismatic needles. These 
crystals contain pyridine, which may be removed by washing 
with alcohol and drying at i6o° C. EUagic acid may perhaps 
dissolve in other basic liquids such as quinoline or solutions of 
urea. Solutions of ellagic acid in alkali are yellow, and give a 
crystalline precipitate when diluted with hot alcohol and acidified. 
The most important colour test for ellagic acid is Griessmayer's 
test, carried out by adding nitric acid containing nitrous acid and 
subsequently diluting. A blood-red coloration is given, once 
thought characteristic of eUagic acid, but shown by Perkin to be 
also given by an allied substance, flavellagic acid, which contains 
five hydroxyl groups. 

Leather tanned with bloom-yielding materials naturally contains 
ellagic acid, the presence of which is often desired, as it gives some 
weight and solidity to the product. In order to extract it the 
leather should be thoroughly freed from fats and water-soluble 
matter in the usual way, the residue then well dried, and finally 
extracted with hot pyridine. EUagic acid so obtained was found 
by Nierenstein to be identical with the synthetical product. 

Most solutions of catechol tannins are decomposed by long 
boiling, particularly with acids, or by heating under pressure. 
A reddish precipitate is obtained totally different from bloom, 
and to which the name of " phlobaphenes " or " reds " is usually 
given. These phlobaphenes appear to be present to some extent 
ready formed in the solutions capable of yielding them. If 
alcoholic or very concentrated aqueous tannin extracts are 
poured into cold water a red precipitate is sometimes formed 
which will not redissolve. Phlobaphenes appear to be produced 
from the catechol tannins by condensation of molecules through 
anhydrisation or oxidation, or both. Nothing exact can, how- 
ever, be said on this point, since the constitutions of catechol 
tannins and phlobaphenes are ahke unknown. Alcohol, dilute 
alkalies, sulphites, and borax dissolve phlobaphenes, and attempts 
have been made on these lines to render phlobaphenes available 
for tanning. One of the most successful of these efforts has been 
the sulphiting process of Lepetit, Dollfus, and Gausser, who heat 
with bisulphite under pressure. Some phlobaphenes are in- 
soluble in water even at boiling temperatures, though they 
dissolve to some extent in the presence of sugars, tannins, etc. 
This is probably a case of the well-known colloidal phenomenon 
of peptisation. Other phlobaphenes are more soluble in water, 
probably because they are more hydrated. Such substances are 



THE CHEMISTRY OF THE TANNINS 343 

able to precipitate gelatin and combine with hide, and must be 
regarded as difficultly soluble tannins. It is on this account 
that the determination of insoluble matter in tanning extracts 
becomes so difficult to decide upon. A sharp distinction between 
difficultly soluble tannin, and strictly insoluble and consequently 
useless matter, cannot be drawn. 

This chapter could not be concluded without some mention 
of the artificial tanning materials of the Neradol class. Many 
of these are on the market and in wide use. The actual details 
of preparation are largely manufacturers' secrets in the case of 
both Continental and British products, but the principle under- 
lying their preparation is generally the same. It has long been 
known that phenol-sulphonic acids would condense with form- 
aldehyde, or other aldehydes, in the presence of acids to form 
complex bodies of higher molecular weight. These bodies are 
usually capable of tanning, but owing to their insolubility in 
water are of no practical use. Stiasny's important discovery 
was that by a suitable process water-soluble tanning substances 
could be prepared. For example, 10 kilos, of phenol is heated 
at 105° to 106° C. with its own weight of concentrated sulphuric 
acid for two hours. The mixture is then cooled to 35° C, and 
about 4I kilos, of 30 per cent, formaldehyde gradually added 
over a period of three hours without allowing the temperature to 
rise. This latter precaution is absolutely essential, and is to 
avoid the production of insoluble bodies. After the form- 
aldehyde has been added the mixture is stirred for a few hours. 
Thus one obtains about 24 kilos, of the crude condensation product. 
In practice phenol is not used, but is replaced by the cheaper 
commercial cresylic acid, a mixture of the three isomeric cresols : 
OH CH, - CH, CH, 



OH 



OH 



Phenol. ^-cresol. w-cresol. ^" 

/-cresol. 

The formation of Neradol D may be formulated as follows, 
according to Grasser : 

OH OH OH OH 

"A:." ? ^.r^^ h/\_ch,— /\h 

hI^CHs 'Hgd^H H^CHs Hgd 'h 

HSO3 HSO3 HSO3 HSO3 

Dicresylmethanedisulphonic acid. 
Neradol D. 



344 PRINCIPLES OF LEATHER MANUFACTURE 



Neradol N and Neradol ND are similar products prepared from 
naphthalene sulphonic acids ; Ordoval G is prepared from higher 
hydrocarbons, principally retene. All these bodies must be 
neutralised to a suitable degree with alkali before they are ready 
for use. 

Neradol D is a substance of brown colour similar in appear- 
ance to ordinary tanning extracts, and has a characteristic smell 
of phenol. It is completely soluble in water, and the organic part, 
but not the sodium salts produced by neutralisation, in alcohol and 
in ether. The ordinary fat solvents, with the exception of ether, 
exert no solvent action. The aqueous solution is usually acid to 
methyl orange, and gives, according to Crasser, the following 
reactions : with barium chloride, a white precipitate insoluble in 
nitric acid ; a deep blue colour with ferric salts ; no precipitate 
with bromine, or with formaldehyde and hydrochloric acid ; 
complete precipitation with gelatin ; a distinct precipitate with 
aniline and hydrochloric acid. Its behaviour is thus very much 
like that of a pyrogallol tannin. It will be noticed that it behaves 
with regard to aniline in the same way as sulphite-cellulose 
extract. Neradol D in acid aqueous solution is electrically 
negatively charged. The practical use of Neradol D cannot be 
described here, but attention may be drawn to its power of 
solubilising phlobaphenes, and its probably consequent property 
of lightening the colour of leather tanned with quebracho and 
similar materials. 

The reactions of Neradol N and Ordoval G are summarised in 
the following table, due to Grasser : 



Gelatin . 



Ferric chloride 
Barium chloride 

Bromine water 
Silver nitrate . 
Aniline and HCl 

Formaldehyde and 
HCl . 



Neradol N. 

Ppt., somewhat sol- 
uble in excess of 
tannin. 

No colour. 

White ppt., insol- 
uble in HNO3. 

No reaction. 

No reaction. 

Ppt., soluble on 
heating. • 



Ordoval G. 

Moderate pptn. in 
fine flocks. 

Slight darkening. 
Turbidity. 

No reaction. 
Opalescence. 
Brownish-black 
ppt. 

No. ppt. 



The failure of the iron reaction is probably due to the absence of 
hydroxyl groups. 



CHAPTER XX 

THE SAMPLING AND ANALYSIS OF TANNING MATERIALS 

Although the analysis of tanning materials falls more properly 
within the scope of a book for chemists than one intended 
primarily for tanners, and though it has been treated at con- 
siderable length in the Leather Industries Laboratory Book and, 
more recently, in Leather Chemists' Pocket Book,^ a slight sketch 
must now be given of the methods in general use, since it is of 
great importance that at least the principles on which they are 
based should be understood by all to whom they are of practical 
interest ; but the method has become too complex and detailed 
to be within the scope of the ordinary tanner, and those who 
wish to pursue it will find full details in the books quoted, and in 
the Journals of th^ respective Associations, of which it is open 
to anyone to become an Associate who is interested. 

It must specially be insisted on, that absolute adherence in 
every detail to the methods given is essential to obtaining con- 
cordant results, and little points of manipulation which appear 
in themselves unimportant are frequently the result of long 
experience and careful discussion. The members of the Society 
of Leather Trades' Chemists especially are bound by their rules 
to make note in their analytical reports of any deviation, however 
small, from the prescribed process. 

The first step in the analysis of any material is to draw a 
sample truly representing the bulk, which usually depends on 
the taiiner himself or his agent, and which is often by no means 
easy, while failure to accomplish it is probably the cause of more 
errors and disputes than any inaccuracy of the method of analysis 
itself. In very many cases chemists are blamed for discrepancies 
which really exist in the samples supplied to them, and they 
can only hold themselves responsible for the accuracy of their 
analyses when the sampling has been done strictly according to 
the rules prescribed by their Associations. On this account, all 
important samples should be drawn in the presence of a principal 
or some other responsible person. 

In liquid extracts, the thorough mixing of the liquid is of the 
greatest importance. Most extracts contain a portion of " diffi- 
^ E. & F. N. Spon, 1919. 
345 



346 PRINCIPLES OF LEATHER MANUFACTURE 



cultly soluble" tannins (see p. 351), which slowly settle to the 
bottom or adhere to' the sides of the cask, from which such 
expedients as merely rolling a full cask are quite inefhcient to 
dislodge them. In fact nothing but taking the heads out of a 
sufficient number of casks, and actually stirring them with a 
suitable plunger, which should be specially applied to the sides 
and bottom, or emptying the entire contents of the casks into a 
tank in which the whole can be adequately mixed, is really 




GiuUTneter^. 
Inches. 



n 



9 ro 



2S 



30 



Fig. 75. — Kathreiner's Sampling Tools. A, strong cross-handle ; B, guard- 
disc ; C C, brass tube sharpened at C ; D, brass or wooden plunger. 



thoroughly reliable, though at times it is necessary to be content 
with less satisfactory methods. In any case, when it is probable 
that samples must be submitted to more than one chemist, the 
whole should be drawn at once, thoroughly mixed and divided, 
and sealed in separate bottles, and in dividing a sample the 
same care must be taken to ensure complete mixture as in 
drawing the original sample. 

Solid and pasty extracts, such as quebracho, cutch, and gambler, 
are still more difficult to sample fairly, as the outside is almost 
invariably much drier than the interior. Generally the only 
way is to select such portions as are thought fairly to represent 
the bulk, to chop them into moderately small pieces, mix and 
seal in an air-tight tin, leaving it to the chemist to draw from 



ANALYSIS OF TANNING MATERIALS 347 

these the smaller sample required for analysis. Gambier is best 
sampled with a tubular tool like a cork-borer, designed by Mr 
Kathreiner (fig. 75), which must be passed completely through 
the bale, or the cylindrical sample of gambier cannot be with- 
drawn. The same tool may also be used for sampling sumach 
in bags if the damage to the bag is not objected to. If such a 
tool is not available, the only fair way to sample gambier is to 
cut slices completely through the bale with a clean fleshing-knife. 
In any case it is of the utmost importance that the sample once 
drawn should be mixed as rapidly as possible, and at once 
enclosed in an air-tight box or jar, sealed, and labelled. 

Dry tanning materials, such as bark and valonia, require 
judgment in selecting samples which fairly represent bulk. If 
they are of a nature which do not really separate into dust and 
fibre, a good method is to grind a sufficient quantity in an ordinary 
bark-mill, and, after well mixing, to draw the sample from the 
ground portion. In other cases it is best to empty a sufficient 
number of bags one upon another in layers on a smooth floor, 
and to divide by " quartering " ; that is, by halving the total 
quantity down to the floor, and halving the half again till the 
part is reduced to a suitable sample. In such materials as valonia 
and divi-divi, the dust or beard is usually much stronger than 
the average of the pods or cups. 

The same sort of precautions are required in drawing the 
still smaller sample required for analysis from the larger original 
sample, but these are sufficiently detailed in the directions of the 
chemical text -books. As materials usually require finer grinding 
than can be managed with the mills employed in the tannery, 
a suitable mill may be provided, and one of the simplest, at a 
moderate price, is the No. 4 drug-mill made by A. Kenrick & 
Sons, Limited, West Bromwich (fig. 76), but this, and the final 
drawing of a small sample for analysis, is usually left to the 
chemist. Coffee miUs are seldom strong enough for the purpose, 
but if nothing better is available, the sample must be thoroughly 
dried before grinding, and its loss of weight noted and taken into 
account in calculating the analysis, care being taken that the 
sample after grinding is so- preserved that it cannot re-absorb 
moisture. Valonia, myrobalans, and even barks, may before 
grinding be broken with a flat -faced hammer on a thick cast- 
iron plate, with raised edges to prevent loss from flying fragments. 
Barks in " long rind " may be bound in bundles with wire 
and sawn into short lengths, or the sawdust taken for analysis. 

As to the analytical process itself, almost innumerable methods 
have been proposed. Dekker in his book on Die Gerbstoffe 



348 PRINCIPLES OF LEATHER MANUFACTURE 

describes more than eighty, but of these only two have kept their 
place as of practical value— the " hide-powder " method for all 
commercial analyses, and the Lowenthal volumetric method for 
some purposes of tannery control. The earliest attempt at 
quantitative analysis of tanning materials seems to have been 
made by Biggin in 1800, and improved by Davy in 1803 or 1804. 
The tanning infusion was precipitated with gelatin, and the 




Fig. y6.- — Kenrick's Drug-mill, 



precipitate dried and weighed after more or less washing, and the 
tannin reckoned as half the weight of the precipitate. Com- 
plete washing was difficult or impossible, and it was found that 
different proportions of tannin were carried down in the preci- 
pitate by different concentrations of the solutions. Almost 
innumerable metallic salts will precipitate tannin, especially 
those of the heavy metals, and many methods have been founded 
on this fact, but the precipitates are of inconstant composition, 
and as the tannins are a class, differing rather widely in chemical 
constitution, and not a single chemical individual, as was at one 
time supposed, it follows that even if an actual tannin-salt could 
be obtained, it would be of different proportions for different 
tannins. In fact several of the methods with metallic salts will 
give fair results if only one species of tannin is to be determined. 
A more serious trouble perhaps is that all natural tanning materials 



ANALYSIS OF TANNING MATERIALS 349 

contain not merely the tannins themselves, but their phenolic 
derivatives, such as gallic acid, which are incapable of tanning, 
but which are mostly precipitated by the metallic salts. The 
necessity therefore arises of removing these bodies by a separate 
operation, which is not generally possible. 

Much more useful and rapid methods are those dependent on 
the oxidation of the tannin by a volumetric solution of potassium 
permanganate or some other oxidising agent, and the best of 
these still seems to be the original Lowenthal method invented 
in i860, and subsequently improved by Lowenthal and others. 
This depends on the oxidation of the tannin in very weak solution 
in presence of a known quantity of sulphindigotic acid till the 
deep blue of the solution is changed to a clear yellow. The 
sulphindigotic acid serves not only as an indicator, but as a 
regulator of the oxidation, only those substances being attacked 
which are more readily oxidisable than the indigo itself. As, 
however, these include gallic acid and other phenohc derivatives 
generally present, it is necessary to remove the tannin by hide- 
powder, or by precipitation with gelatin, and make a second 
determination of the remaining " non-tannins " and thus 
obtain the permanganate-value of the tannin by difference, 
that of the indigo being deducted in each case. The process 
sounds complicated, but where a number of determinations are 
to be made it is the quickest known, as each titration only takes 
a few minutes, and the end-point is quite sharp, while the removal 
of the tannin, especially with a gelatin solution in presence of 
salt and sulphuric acid, is also very rapid. It has the advantage 
that it can be done in extremely dilute solution and is thus 
especially adapted to the control of weak liquors but, like all 
other purely chemical methods, it gives different results with 
different tannins. Tfie permanganate-value of the different 
tannins may be determined by comparison with the hide-powder 
gravimetric process, and the permanganate solution is now 
generally standardised by a comparison with a solution of pure 
gallic acid under the same conditions as the tannin test ; but the 
chief use of the method is for the relative determination of a 
series of liquors made from the same materials, as in the control 
of suspender liquors, or of the exhaustion of spent tanning 
materials. Full details of the precautions required in its execu- 
tion may be obtained from the text-books already named, and if 
the standard solutions are provided, it should not be beyond the 
ability of an intelligent foreman to carry out the actual deter- 
minations for the purposes just suggested. 

The hide-powder gravimetric process is of a quite different 



350 PRINCIPLES OF LEATHER MANUFACTURE 

character, and is really of the nature of a miniature tannage 
carried out on a laboratory scale and in a very limited time. It 
is therefore applicable to all tannins, and determines the actual 
quantity of matter present which is absorbable by hide, but of 
course does not give the relative value as affected by colour or 
by the quality of the leather produced. Being an empirical 
process, it is necessary to carry out the experiments with absolute 
uniformity to ensure comparable results, and though it has now 
been perfected so that very concordant results are obtained, 
there are still several sources of error which have not been com- 
pletely eliminated, one of which is the difference- of absorptive 
power of different batches of hide-powder, even when the 
greatest care is taken to ensure a uniform product. For this 
reason the powder of a single maker is employed by all the 
chemists of the Society, and no batch is allowed to come into 
use till it has been practically tested and passed by a committee 
appointed for the purpose. 

The principle of the method is that a weak solution is made 
from a known weight of the tanning material in a given volume 
of water, and, after filtration, the dissolved matter it contains is 
determined by evaporating 50 c.c. of it to dryness and weighing 
in a tared basin. As this includes all soluble matters present, it 
is necessary to remove the tannin from a portion of the solution 
by shaking with a known weight of a carefully washed and lightly 
chromed hide-powder, which can be proved to give no appreciable 
soluble matter when shaken with pure water, and to determine 
the " non-tannins " by evaporation of a second 50 c.c. under 
similar conditions. The " moisture " in the original sample is 
now determined by dr5dng a weighed portion (generally at 
100° C. in vacuo), or alternatively, in the case of extracts, the 
" total solid matter," by evaporating 50 c.c. of the well-mixed 
unfiUered solution ; and from these data the following results are 
obtained by calculation: — 

" Tanning matters absorbed by hide ". (=total soluble less 

" non-tannins "). . 
" Non-tannins " (by direct weighing of the dried detannised 

solution). 
" Moisture " (by loss on drying the original substance, or by 

deduction of total solids from total weight used, or from 

100 per cent.). 
" Insolubles " (by deduction of the above from 100 per cent.). 

It is curious that the greatest discrepancies between different 
analysts have often occurred in those points which seem most 



ANALYSIS OF TANNING MATERIALS 351 

easy of direct determination, such as the amount of moisture 
present in the original sample. This may be partly accounted 
for by the difficulty of preserving a sample without loss or gain 
of moisture, but it also shows the necessity of absolute uniformity 
in the temperature of drying, the degree of vacuum used, and all 
other conditions which may affect the result. 

As has been observed, the amount of " non-tannins " found is 
varied to a certain extent by the character of the hide-powder, 
although the tanning matters are always completely absorbed, as 
is shown by the detannised liquor failing to give any cloudiness 
with a solutio'n of slightly acidified and salted gelatin. The fact 
is that a proportion, sometimes considerable, of the phenolic 
non-tannin derivatives, which are always present in tanning 
materials, are absorbed by the hide-powder, though the greatest 
care has been devoted to reducing this to the smallest possible. 
These non-tannins are also absorbed by hides in the actual 
tanning process, so that they are not commercially valueless to 
the tanner, but they are only loosely combined, and can be 
removed by sufficient washing, so that, scientifically, they should 
not be regarded as tanning matters. J. A. Wilson has recently 
attempted to overcome this error by washing the hide-powder 
after use in detannising, and so obtaining what he calls " the 
true tanning value," and this often differs to an unexpected 
degree from the results of the official method. This will be 
discussed in detail on a later page. In any case it is desirable to 
approximate the results of analysis as much as possible to the 
scientific standard, and there is no doubt that further efforts 
will be made in this direction, but, in the meantime, the results 
of the official method have become of great commercial im- 
portance, and changes cannot be rashly made. 

A further point on which the present method is unsatisfactory, 
but to which the same remarks apply, is that of the estimation of 
so-called " insolubles " in extract. The greater part of this is not 
really insoluble or useless to the tanner, but consists of diffi- 
cultly soluble tannins which separate from the hot solution as it 
cools in a .very finely divided and colloidal state, so that it is 
almost impossible to remove them by filtration. When the 
extract is used in tanning much of this ultimately passes into 
solution and is absorbed by the leather, and the. writer has 
experimentally tanned sheep-skin quite satisfactorily vnth. the 
separated " insolubles " of quebracho extract by merely drum- 
ming with cold water. On filtration, the amount which passes 
through is not only affected by the texture of the filter, but by 
the exact temperature of the solution, and the length of time 



352 PRINCIPLES OF LEATHER MANUFACTURE 

during which it has been cooling, and it is almost impossible for 
two' chemists to obtain identical results, while solutions which 
are transparent to transmitted light may contain lo per cent, of 
matter which may be removed by closer filtration. It was 
hoped that these difficulties would be largely overcome by the 
use of the Berkefeld " candle-filter," which consists of a hollow 
cylinder of finely divided silica cemented by some adhesive, 
and through which the liquid is forced by atmospheric pressure. 
This did not by any means wholly overcome the difficulty, and 
during the war it was impossible to obtain " candles " of 
the same constant texture, and recourse had again to be made 
to filter-paper with the assistance of powdered kaolin. This 
introduces a fresh difficulty, since some paper itself absorbs a 
small amount of tannin, which must be allowed for. It is now 
stated that satisfactory " candles " can be obtained. Really 
insoluble matter, such as sawdust and fragments of vegetable 
tissue, are rare in well-manufactured extracts, and are easily 
removed, while the so-called " insolubles " are mostly completely 
soluble at a higher temperature or by the addition of a little 
ether, and it has been suggested that a filtration should take 
place under these conditions, when in most cases no " insoluble " 
would be reported. As, however, many extracts are rendered 
wholly soluble by additions of bisulphite and in other ways, it is 
necessary that these should be distinguished, though perhaps a 
measurement or statement of the turbidity at laboratory tem- 
perature might be sufficient. Clear-soluble extracts are largely 
used for bleaching purposes, but as tanning agents they give 
porous and light-weighing leather, while the difficultly soluble 
tannins greatly contribute to weight and sohdity. 

Although for many purposes the colour of leather is quite un- 
important, light colour has become a fashion with a money value,; 
and where leather is to be dyed a clear shade, it is essential. 
The effect of a tanning material in this respect is best determined 
by tanning small pieces of prepared calf- or sheep-skin for a 
determined time in a solution of determined strength. Details 
of the execution of such experiments are given in the laboratory 
text -books. They do not, however, give numerical results which 
can be inserted in a contract ; and neither samples of extract 
nor of leather can be kept unchanged for a length of time, so 
that a direct optical measurement of the colour of the solution, 
which will give definite figures, has become very customary, as 
although the colour of the liquid bears no very definite relation 
to that of the leather which wiU be produced, it serves as a mode 
of comparison of the bulk delivered with the original sample on 



ANALYSIS OF TANNING MATERIALS 353 

which it was bought. The method usually adopted is to com- 
pare the colour of the filtered analytical solution in a half -inch 
(now I cm.) glass-sided cell with that of the graded glasses of the 
Lovibond tintometer till a match is obtained. As the analytical 
solution varies somewhat in strength of tannin, the result is 
usually calculated to that which should be given by a solution 
containing 0-5 per cent, of tanning matter as shown by the 
analysis. This method is imperfect in many respects, since the 
colour of the glasses, though visually the same as that of the 
liquor, is really differently constituted, as will be seen at once 
on comparing the two by a spectroscope. While the spectrum 
of the liquid shows a regular gradation from the blue end of 
the spectrum, which is most absorbed, to the red, that of the 
glasses is irregularly banded, especially in the green, which is 
specially absorbed by the red glass. If the two colours were 
really identical, their match would not be affected by the quality 
of the light, which would influence both in the same way. As it 
is, however, if both be looked at through a coloured glass they 
will usually cease to match, and similarly on a clear day with a 
blue sky the match will not be the same as on a dull day, or 
with a yellow fog, or by artificial light. It was found that even 
using the identical set of Lovibond glasses a different match 
was required in London to that obtained under the clearer sky 
of Paris. For the same reason a double thickness or strength 
of the liquid is not matched by doubling the glasses, and the 
calculation from the approximately 0-4 per cent, analytical 
solution does not give the same result as one obtained direct 
from a 0-5 per cent, solution specially made after the analysis is 
completed. A further difficulty arises from the fact that the 
colour of solutions usually darkens on standing. Attempts have 
been made to get over the differences in daylight by the use of 
artificial light, either of the Welsbach burner or from a tungsten 
or Nernst electric light, but besides the difficulty of keeping these 
lights really constant, there is the still more formidable one that 
the Lovibond glasses do not seem always to agree perfectly in 
different sets, although the greatest care is certainly taken to 
make them accurate. 

It is desirable that some method should be found either to 
measure direct by the colours of the prismatic spectrum, or to 
refer to those of chemical solutions which could at any time be 
accurately reproduced in the laboratory, but neither is easy, 
and it is difficult to find a mode of statement which would be at 
the same time accurate and definite, and convey to tanners any 
clear idea of the actual colour. For the first purpose it is difficult 

23 



354 PRINCIPLES OF LEATHER MANUFACTURE 

to find three colour solutions suitable and at the same time 
sufficiently permanent, and for the second, in addition to the 
difficulty of intelligible staternent, costly and very accurate 
apparatus would necessarily be required. A special difficulty is 
to find a red solution really permanent to light. Coal-tar colours 
are almost impossible to obtain of the requisite purity, and usually 
fade or change, and even cobalt chloride becomes brown on 
continued exposure to light. The writer has already spent 
much time on research to overcome these difficulties, and with 
considerable success, and hopes to publish details shortly. 
It is complicated by the fact that the colours obtained 
by superposing glasses are complementary to those given by 
direct mixture of the light of corresponding colour. Abney's 
small book on Colour Measurement,'^ or the more recent one of 
Luckiesh ^ on the same subject, may be referred to for further 
information. 

1 Kegan Paul, Trench, Triibner & Co. (Out of print.) 
^ Luckiesh, Colour and its Applications. Constable. 



CHAPTER XXI 

PRINCIPLES OF THE VEGETABLE TANNING PROCESSES 

The processes employed in the production of leather with the 
vegetable tanning materials vary extremely according to the 
class of leather which is being produced, both in the materials 
selected and in the time required. In sole leather tanning, 
where thick hides are used, and where diffusion is the only force 
acting to carry the tannin into the hide, many months are 
frequently needed, while with thin skins, and with the aid of 
mechanical motion, which circulates the tanning liquid between 
the fibres, the process is often complete within a few hours. 
" Drumming " or other similar mechanical means are now 
largely used on the heavier leathers, with great consequent 
shortening of the process. Differences in the strength of the 
liquors, according to whether hard or soft leathers are to be pro- 
duced, and the mutual action of the acids naturally present in 
the liquors, a-nd of the tan, have also a determining effect upon 
the quality of the product ; and much depends on the special 
properties of the tanning materials selected. 

The simplest form of tanning in principle is probably the old- 
fashioned method of sole leather manufacture. For this purpose 
the hides are usually " rounded " or trimmed after liming, un- 
hairing, and fleshing, so that the most valuable part, the " butt," 
can be tanned separately from the " offal," which frequently 
gets cheaper materials and a much shorter tannage. The butts 
are usually washed in water to remove a portion of the lime, 
considerable care being required at this stage to avoid carbona- 
tion and fixation of chalk by free carbonic acid, or hydric 
calcium carbonate (temporary hardness) in the water employed, 
or by the free carbonic acid of the air. This somewhat primitive 
process can at best only remove a small portion of the lime, 
since so long as the lime remains in the caustic condition it is 
very obstinately held by the hide-fibre. Advanced tanners now 
employ weakly acid baths, in addition to washing, in order 
to produce more complete - deliming, and this effects a very 
considerable improvement of colour and economy of tannin in 
the early liquors. The use of weak organic acids (formic, acetic, 
lactic, or butyric, free from iron) or of boric (boracic) acid in 

355 



356 PRINCIPLES OF LEATHER MANUFACTURE 

solution of about 4 lb. per 100 gallons, in which the butts are 
kept in motion, are among the safest and most satisfactory ways 
of removing surface-lime and improving the colour, but even the 
stronger mineral acids may be used successfully with caution 
(see Chapter XIV.).' Dilute solution of sulphurous acid, made 
by burning brimstone (p. 24), is also very satisfactory. 

Acids should never be used of such strength as to materially 
swell the butts, and the great advantage of the " weak acids " is 
that from their low ionisation they can be used more freely, while 
" strong " acids m-ust be added in very small and repeated doses. 
For further particulars see Chapter XIV. 

If hides, before bringing into the liquors, are exposed either to 
the carbonic acid of the air or that of " temporary " hard water 
a precipitate of lime carbonate is formed in the skin which is 
much more difficult than free lime to remove in acid deliming. 
A point of great importance is to keep the goods from the time of 
unhairing till they go into the liquors under water in which there 
is always a trace of caustic lime, or which at any rate are free 
from carbonic acid. In deliming sole leather with acids it is 
best to give the full dose of acid required at once, and not gradu- 
ally, so that it may act most powerfully on the exterior, and 
remove any carbonates present, before it penetrates to and 
becomes neutralised by the excess of lime in the interior. This 
is exactly the reverse of what is advisable with dressing leather, 
where the object of the tanner is to remove lime as uniformly 
and completely as possible, without excessive acidity of any part. 
Of course hides should not, even in the case of sole leather, be 
allowed to go into the liquors while any acid swelling of the 
surface remains, but this will soon disappear if the goods are 
suspended for a time in cold water after deliming, unless excess 
of acid has been used [cp. p. 203 et seq.). 

Whether acid be used or not, the butts are now usually sus- 
pended in deep pits containing old and nearly exhausted tan 
liquors. These liquors contain a certain amount of lactic and 
acetic acids, derived by fermentation from the sugary matters of 
the tanning materials, and also, in some cases, weak acids origi- 
nally present in the materials themselves. The presence of 
these acids is most important to successful tannage, and their 
effect is twofold: in the first place, they neutralise and remove 
any lime which still remains in the butts ; and, secondly, 
they bring the butt into a slightly acid condition, which is 
necessary to tannage, and in which it remains plump and swollen 
in the liquors, while the tannin gradually penetrates and tans 
the fibre. If, as frequently happens, especially in modern yards 



VEGETABLE TANNING PROCESSES 357 

where extracts are very largely used, the natural acid of the 
liquors is not sufficient for this purpose, the lime combines with 
the tanning matters, and the butts either become discoloured at 
once, or darken by exposure and oxidation; when they come to 
be dried, while the pelt remains flat and insufficiently swollen. 
To avoid this trouble, resort is sometimes had to artificial acidi- 
fication of the liquors. As a general rule, it may be stated that 
it does not answer to mix the stronger mineral acids directly 
with the liquors, but lactic and acetic acids may be used, or even 
oxalic acid may be added to the suspenders in such quantities 
as to precipitate and remove the lime which they contain, setting 
free the organic acids with which it had been combined. The 
use of oxalic acid should never be pushed further than this, as it 
has a most powerful swelling action on the hide, and goods 
which are too much swollen by acids tan dark and brittle. The 
acidity of such liquors is usually determined by Procter's lime- 
water test,i i.e. the volume of saturated lime-water which can be 
added (with constant stirring) to 10 c.c. of the clear filtered 
liquor without producing a permanent cloudiness. This test 
does not with any accuracy indicate the swelling power of 
the liquor, which depends mainly on its hydrion concentration 
(p. 103), and is much influenced by the presence of neutral 
salts, but it does directly give the amount of lime which 
can be safely carried into the liquor without the risk of pro- 
ducing stains. Many of the acids present in the liquors, though 
too " weak " to cause swelling, are still capable of dissolving 
lime. If lime be present beyond this point, it does not necessarily 
produce immediate stains, which are due to the oxidation of the 
tannin salts formed by exposure to the air, and if this be avoided, 
the lime may ultimately be got rid of in the more forward liquors, 
and the hides may remain of good colour ; and this was usually 
what was aimed at before acid-deliming was introduced. The 
greatest danger of stains arises when the hides are allowed to 
touch each other in the suspenders. In this case white patches 
are at first formed where the tan -liquors have not access, and 
these may ultimately colour properly when the hides are moved, 
but their edges, where there is a little tan and a great excess 
of lime, usually oxidise, and remain as permanently dark map- 
like outlines. For this reason it is most important that the hides 
should be freely moved for the first few hours after coming into 
the liquors, either by hand, or by suspending them to a mechani- 

^ Before applying this test to liquors containing oxalates, it is necessary 
to remove the oxalic acid by the addition of a known volume of calcium 
chloride solution before filtration. 



358 PRINCIPLES OF LEATHER MANUFACTURE 

cally moved frame. If the butts are hung on sticks, these may be 
allowed to rest on bearers of about 2-inch by 3-inch section at the 
sides of the pits, with occasional crossbars to keep them parallel, 
and are either suspended by iron rods from beams above, or sup- 
ported on rollers, which should be brass or hard wood, motion being 
given by an eccentric or crank with about 6 revolutions per minute, 
and a stroke of 6 inches or 8 inches, space for which must be allowed 
in the pits. The motion considerably increases the rapidity of the 
tanning, and renders possible a better exhaustion of the liquors, 
which should usually be run away after use on the green goods. 
The pits are best arranged in a continuous series, the top of each 
pit being connected by a wooden trunk with the bottom of the 
next weaker, all the liquors being run in at the head end, and all 
the green goods brought in at the other and gradually advanced. 

If the goods are brought into suspenders without previous 
deliming, the first action which takes place is the neutralisation 
of the lime at the expense of the weak acids of the liquors, or, if 
these are not present in sufficient quantity, at that of the tannin, 
which forms mostly insoluble compounds with the lime, which 
readily oxidise, and produce dark stains if exposed to the air, 
though, if protected from it, they ultimately dissolve in the more 
acid and stronger liquors. If the goods colour quickly (with 
valonia, to a lemon-yellow colour), this may always be suspected. 
At the same time the plumpness due to the lime disappears, 
and the goods become soft and spongy, and, if pressed at this 
stage, do not readily regain their plumpness, and pressure-marks 
and drawn grain are apt to remain permanently. If the liquors 
are of proper acidity, plumpness is gradually regained by gentle 
acid swelling, but this should not take place before the surface 
is tanned and no longer susceptible to the effect of acids, or the 
grain will darken and be liable to crack when dry. When hides 
are swollen with strong acids, as is sometimes done in the States, 
this is always after the surface is fairly coloured, and even then 
a dark layer is apt to form below the grain, where the tannin 
has not had- time to penetrate. 

If new materials have to be used for making suspender- 
liquors, the less astringent tans, such as gambler and myro- 
balans, are most suitable, and the synthetic tans, such as 
Neradol, are now much used, as they are very light in colour, 
and usually rather acid. The colour which is given in the 
suspenders largely determines that of the finished product. 

After the hides have remained from ten days to a fortnight in 
the suspenders, they are usually laid in pits called " handlers," 
which are worked in series of six, eight, or ten pits, containing 



VEGETABLE TANNING PROCESSES 359 

the same number of packs of goods. The weakest liquor from the 
youngest pack is run to the suspenders daily, a new and stronger 
liquor is run to the pit, which now becomes the head of the 
series, into which the oldest and most tanned pack of butts is 
moved ; and the next takes its place and liquor, and so on down 
the series, the youngest pack finally occupying the place which 
had previously been taken by the last but one. In this way 
each pack receives a change of liquor of regularly graduated 
strength ; and during the time which it remains in the handlers, 
passes from a strength of perhaps 20° Bkr. (sp. gr. 1-020) to one 
of about 40° Bkr. (sp. gr. 1-040). ^ During this part of the 
process the butt is completely or nearly coloured through, and 
is then ready for the " layers." 

In some yards suspension is continued through the handler 
period, and sometimes elaborate systems of overhead cranes have 
been fitted to move the packs from one pit to another. These 
no doubt save labour, but they have the disadvantage of not 
changing the position of the butts to each other, and it is not 
certain that the economy repays the much greater cost of the 
installation. A good method of handling is to attach cords to 
two corners of the butts. These must be long enough to reach 
the bottom of the pit, and terminate above in rings Or loops 
which are placed over pegs at two corners of the pit. Two 
men pull the butts over into the next pit by these cords, while 
a third man puts them down evenly in the liquor with a 
pole. The old method of handling with sharp hooks is much 
slower, and requires considerable skill to avoid scratching the 
grain. 

In the forward handlers, dustings of ground bark or other 
tanning material are very frequently given, not merely to keep 
up the strength of the liquors, but to separate the goods from 
each other, and the layers only differ from these in having" much 
heavier dusting, stronger liquors, and being allowed to remain 
undisturbed for greater lengths of time, ranging from a week up 
to a month or even six weeks as the tannage progresses. The 
handler-liquors are principally from the old layers, though they 
are frequently made up with v/eak liquors from the leaches, and 
strengthened with extracts or gambler. Goods in the suspenders 
and handlers tan very rapidly, and require large volumes and 
frequent changes of weak liquors, which the layer-pits can hardly 
supply. A well-thought-out system of running the liquors is 
one of the most important things in a heavy-leather tannery, 

^ In modern yards quite double these gravities are frequently used, the 
head-liquors being correspondingly stronger. 



36o PRINCIPLES OF LEATHER MANUFACTURE 

and the pumping should never be entrusted, as it often is, to an 
ignorant, and possibly stupid, labourer. 

Very varied materials are used in the manufacture of sole 
leather. Oak-bark is one of the oldest, and as regards quality . 
one of the most satisfactory, but it is costly, not only on account 
of its weakness in tannin but from the light weight of leather 
which it gives. Valonia has been one of the favourite materials, 
giving heavy weight and a solid leather, in which it deposits a 
great deal of bloom ; but its place has been largely taken by 
oakwood, chestnut-wood, mimosa and quebracho extracts, 
which save the cost of grinding and extraction of solid materials. 
These extracts are now very largely consumed, principally in 
strengthening the layer-liquors ; the great object being not only 
to lessen the cost in material, but to save time, and produce 
greater weight and firmness. The layer-liquors in some yards 
where extract is used reach strengths of even 120° to 150° Bkr. 
(sp. gr. I -12 to 1-15), while in pure oak-bark yards it is difficult 
to get above 30° or 35° Bkr. ; and even these figures are only 
reached by repeatedly strengthening the same liquor, in which 
large quantities of non-tanning substances accumulate. The 
opinion of the most intelligent tanners is, -however, that better 
results are attained by a regular change of liquor, even if the 
apparent strength is less. 

When the leather has remained a sufficient time in the layers 
to have attained all the weight and solidity of which it is capable, 
it is washed up in a clear and somewhat weaker liquor, and is 
ready to be taken to the shed to be dried and finished. This 
simple method has, however, largely given place to others less 
defensible, partly owing to the fact that sole leather is sold by 
weight, and partly to the absurd demand of the shoe manu- 
facturer for a light colour, which he proceeds to stain black, or 
to cover up with a " fake " ! The usual thing now is to suspend 
the butts before taking into the shed in a warm and very strong 
liquor of " bleaching extract " very heavily bisulphited. This 
has the property of dissolving the " reds " or phlobaphenes, to 
which the leather owes a good deal of its solidity as well as its 
colour, and replacing the weight with soluble extract, leaving 
the leather bright coloured, but more porous and less waterproof 
than before. The synthetic tannins may be used as bleaching 
liquors with good effect. Another modern method of weighting 
is to hang the generally lightly tanned and bright-coloured 
butts up till half dry or more, and then to drum them with un- 
diluted extract ; and this drying and drumming is sometimes 
repeated. Of course this completes the tannage and makes a 



VEGETABLE TANNING PROCESSES 361 

firm and heavy-weighing leather, but one with an inordinately 
large percentage of matter which can be washed out with warm 
water. The goods are now taken into the shed to be dried and 
finished. 

If the proportion of free acid in the suspender liquors is as it 
ought to be, it is probably rather advantageous than otherwise 
for a little lime to remain in the interior of the hide, as it keeps 
the pelt in a plump condition during the first stages of colouring, 
quickens the penetration of the tannin, and lessens the tendency 
to " drawn " or wrinkled grain, which arises when the goods go 
into the liquors in a flat or fallen condition. The causes of 
drawn grain are often a little obscure. Of course that case 
needs no elucidation in which the hides are submitted to the 
tanning liquor in a creased or wrinkled condition, which is 
simply fixed and made permanent. This may arise either from 
carelessness in handling the goods before taking into the sus- 
penders, or from the way in which they are slung to the sticks, 
which often draws them into long wrinkles, afterwards difficult 
to remove. Drawn grain in general, however, arises from the 
grain surface becoming tanned and fixed in area, while the sub- 
stance of the hide is in a more extended condition than that 
which it assumes as tannage proceeds. Hides in a flat and un- 
swoUen 'state are thinner, the fibres are slenderer and looser than 
when swollen, and consequently the hide has a larger area. If, 
after the grain is tanned, the substance of the hide becomes 
contracted in the liquor, either by swelling with acids or by the 
direct action of the tannin on the interior fibres, the grain is 
certain to be shrivelled, like the skin of a dried apple. A similar 
effect, produced in a mechanical way, may always be noted 
where a hide has been coloured hanging grain side out over a 
pole, so that the surface is extended at the bend, on which long 
wrinkles are formed as soon as it is straightened. 

A hide in a slightly alkaline condition colours and is penetrated 
by the tan more quickly than one which is acid, though actual 
tannage does not take place till it becomes faintly acid. 
Gambler gives pelt perfectly free from lime a pale buff colour, 
but where lime is present the colour is always reddish and 
much darker, and this coloration does not disappear so readily 
as that with valonia, so that if gambler is to be used in the first 
liquors, care should be taken to remove all lime from the surface. 
The only known tannin which gives no insoluble compound with 
lime is that of the babool pod (sometimes called " gambia pod "), 
which is frequently used in India as a bate, and which would 
probably prove very useful in colouring liquors (p. 329). 



362 PRINCIPLES OF LEATHER MANUFACTURE 

When sole leather first goes into liquors it is generally swollen 
with lime to some extent. If the liquors contain, as they usually 
do, sufficient free acid (acetic, lactic) in addition to the tannins, 
these combine with and neutralise the lime, and the pelt, without 
absolutely becoming flat and thin, loses its firmness, and becomes 
soft and spongy. This is a favourable condition for the absorp- 
tion of tannin, but care should be taken not to aUow the pelt to 
be squeezed or pressed, or water will be squeezed out, and the 
pelt will not easily resume its plumpness. As the tannage pro- 
ceeds, both the tannin and the acid of the liquors penetrate 
deeper into the pelt, the former tending to contract and the 
latter to swell the fibres. Thus a given quantity of acid will 
cause the greater swelling the less tannin is present, and there- 
fore in strong tanning liquors more acid is required. The presence 
of certain products of -bacterial putrefaction has a great but un- 
explained effect in preventing hide from swelling with acids ; and 
in hot weather, much better swelling is obtained by sterilising 
and deliming the hides with carbolic acid or one of the other 
coal-tar products mentioned on pp. 27-29. Boric acid may 
also be satisfactorily used for this purpose with dressing leathers, 
but should not be allowed to get into sole leather liquors, as it 
tends to produce a soft and loose tannage, and from its inorganic 
and indestructible character is apt to accumulate in a yard in 
which it is used. The same reasons render unadvisable its intro- 
duction into any liquors which are to be returned to the leaches 
even in the tannage of dressing leather, though its presence in 
the colouring liquors is otherwise very useful in lessening the 
astringency of the tannins (" mellowing the liquors "), and 
making a fine grain. Its mode of action is by no means clearly 
explained, but is in some way connected with its tendency to 
produce " conjugated acids " {L.I.L.B., pp. 37, 46). 

The so-called " mellowness " of old liquors requires a word of 
comment. It is well known to practical tanners that old liquors 
are much less liable to produce drawn grain, and a harsh surface, 
when used to colour green goods, than liquors, even equally weak, 
which have been made from fresh materials. This is due to more 
than one cause. Most natural tanning materials contain tanning 
matters of varied degrees of astringency and power of attaching 
themselves to the leather-fibre. It is obvious that if a tanning 
liquor is used the most astringent and energetic tannins will be 
first removed from it, leaving those of a milder character. It is 
also known that the presence of neutral alkaline salts of weak 
acids has considerable influence in producing mellowness ; the 
addition, for instance, of sodium acetate has a marked effect. 



VEGETABLE TANNING PROCESSES 363 

This effect is probably due in the first place to the action of 
neutral salts in diminishing the energy of weak acids (see p. 99), 
and secondly to the fact that their bases combine to some extent 
with the tannins ; and that, as was perhaps first pointed out 
by the writer, such tannins are, as it were, partially paralysed 
in their action on hide, since tannin will not combine with hide 
in an alkaline condition (p. 118). Sodium sulphite acts power- 
fully in this way, and may perhaps prove of technical value in 
temporarily diminishing the astringency of liquors in quick 
tannage. Borax has a similar effect, but is too alkaline, and, 
unless used with extreme caution, spoils the colour of the liquors 
by causing oxidation. It is probable that similar causes explain 
the mellowness of palmetto extract, which contains large quanti- 
ties of alkaline salts and non-tannins, and of some extracts which 
have been treated with sulphites, when used undiluted in drum- 
tannage. The addition of free acid wiU generally restore these 
tannins to an active condition. 

J. A. Wilson 1 has recently shown that much of the mildness 
of mild tans is due to the non-tans and weak phenolic acids 
which they contain. Thus quebracho, which is a very rapid and 
astringent tan, with the lowest non -tannins of any in use, 
becomes as mild and slow as gambler, in which the non-tans are 
very high, if a quantity of gallic acid proportionate to the 
difference be added to the quebracho. 

As the tannage proceeds and penetrates further into the hide 
the liquors are used stronger, as the outside, once tanned, is to 
a large extent protected from their action, and it is only by 
continuously increasing the strength of the liquors that more 
tannin can diffuse into the interior, since diffusion only takes 
place from a stronger into a weaker liquor. The liquor in the 
interior of the butts is always exhausted of tannin so long as any 
part of the hide-fibre remains untanned, but as the layer of tanned 
fibre between this and the outside gets thicker, a greater 
difference is required to maintain a reasonable rate of exchange, 
just as a greater head of liquor is required to maintain a flow of 
liquor through an increased number of percolation-leaches. If 
the strength of the liquor outside be allowed to fall off, this 
graduation of strength from the outside to the inside of the butt 
is disturbed, and takes some time to re-establish. As the liquors 
become stronger in tannin they may also become somewhat 
stronger in acid, since, as has been stated, the two act to some 
extent in opposition to each other. The acid-swollen fibre 
absorbs the tannin more slowly than if it were in more neutral 
^ Journ. A.L.C.A., 1920, p. 295. 



364 PRINCIPLES OF LEATHER MANUFACTURE 

condition, but it absorbs it apparently in larger quantity, and at 
any rate makes a firmer, more solid, and less flexible leather. 

It has been mentioned that in the latter stages of the process 
solid tanning materials are generally strewed between the butts in 
the tanning liquor. It may be pointed out that many materials 
vary in their tanning effect, according to whether they are used 
in solid form or merely in liquors. It has been shown by Youl 
and Griffith ^ that such materials as valonia, oakwood and chest- 
nut extracts, and myrobalans, which contain both gallotannic 
and ellagitannic acids, lose strength rapidly when kept in the 
form of liquor, and still more rapidly when heated, ^ the ellagitannic 
acid becoming decomposed with separation of insoluble ellagic 
acid. Now it is just this ellagic acid which, deposited in or on 
the leather, gives weight, solidity, and bloom, and the investiga- 
tion points out not only an important source of loss in the tanning 
industry, but also explains why valonia, which in sole leather 
tannage is known to give hard and heavy leather, can be used 
in large quantities on dressing leathers in Yorkshire, with 
gambier, in the form of liquor, giving a soft and mellow leather 
almost destitute of bloom. In this case the valonia is extracted 
by boiling, and the liquors kept long on the material. If weight 
and solidity are required from the use of such materials, it is 
obvious that they must be brought into immediate contact with 
the leather to be tanned, so that as large a part of the bloom 
as possible is deposited in, and not outside the leather. With 
many other materials, such as hemlock, quebracho, and mimosa, 
which yield no bloom, but " difficultly soluble " tannins (reds or 
phlobaphenes), the same rule holds, since in contact with the 
hides the small proportion of these materials which is soluble 
in the liquors is replaced from the materials as rapidly as it 
is absorbed by the leather, while, when liquors or extracts only 
are used, the greater part of these solidifying and weight -giving 
constituents remain unutilised in the spent tanning materials. 
At the same time the long " layers " afford an opportunity for 
the acetic and lactic fermentations to go on which are the 
principal source of the natural acidity of liquors. It must be 
understood that what are called layers in England are not to 
be identified with the Satze, but rather with the Versenke, of the 
German tanner, the former being layers given in much the same 
manner as was current in England 150 years ago ; in which the 

^ Journ. Soc. Chem. Ind., 1901, p. 428. 

^ For the same reason bloom-yielding materials must be extracted for 
analysis at as low a temperature as possible, and some tannin is always 
destroyed at 100° C. 



VEGETABLE TANNING PROCESSES 365 

leather, with thick layers of tanning material between, is laid 
in the empty pit, which is afterwards filled up with liquor, 
often of a comparatively weak character. In such layers the 
acidification and the solidification of the leather both go on to 
a still greater degree ; the acid formed apparently gradually 
penetrating to the heart of the leather-fibres, and producing a 
solidity and cheesy texture which can hardly be obtained by 
layers of the English kind, which nevertheless have the advantage 
in rapidity and cheapness. 

In drying sole leather, one of the great objects which must be 
aimed at is to remove the dark-coloured liquor, with which the 
goods are saturated, from the surface, and to prevent further 
portions of it from finding their way there from the interior. If 
a strip of filter-paper be allowed to rest with one end in a basin 
containing a little liquor and be placed in a draught of air, the 
exposed end of the paper will rapidly become dark brown or 
black, the liquor which evaporates there being constantly re- 
placed by fresh portions sucked up by capillary attraction from 
the basin. A similar action is constantly seen when filtering 
liquors through paper if the latter be allowed to project above 
the edge of the funnel. Precisely the same effect occurs, perhaps 
increased by the oxidation of the tannins, on the edges and 
other parts of a butt which are most exposed to draughts of air. 
The use of oiling the grain is not only, to a certain extent, to 
protect it from oxidation, but also to check evaporation, and the 
consequent accumulation there of the dark-coloured solids con- 
tained in the liquor. A very similar result is attained by wetting 
the grain side, and allowing a,s much of the evaporation as 
possible to take place from the flesh. 

As the finishing is almost purely mechanical, and scarcely 
comes within the scope of the present volume, a very brief 
sketch must sufiice. The mode of finishing which was formerly, 
at least, in vogue in Lancashire and Cheshire may be taken as 
a type of the best work. (In the present day the various 
methods are so widely known that they have ceased to be local, 
and are varied according to the quality and tannage of the 
goods.) The butts, which in earlier times were largely bark- 
tanned, are taken wet from the pits, and scoured on a rounded 
beam or " horse " with stone and brush till the bloom is com- 
pletely removed, and are then lightly oiled on the grain, half 
dried (" sammed "), laid in pile to temper, and " struck out " with 
' the " pin," a two-handled tool of triangular section. The use 
of this tool has now been largely superseded by Wilson's striking 
machine, in which knives or sleekers (or stones and brushes), 



366 PRINCIPLES OF LEATHER MANUFACTURE 

held in jointed arms, are made to work on the butt, which is 
extended over a slowly rotating cylinder. The object of the 
pinning is not so much to remove bloom or dirt, which has 
been previously effected by the scouring, as to smooth and 
flatten the grain. After further drying a second pinning is 
generally given, and the goods are then twice rolled, first with a 
light weight and somewhat moist grain, and then more heavily 
with the grain nearly dry. This was formerly accomplished by 




Fig. 77. — Offal Roller. 



a sort of box or car, heavily loaded with weights, supported on a 
smooth brass roller of about 5 inches diameter and 9 inches long, 
and manipulated with a long wooden handle on a floor of hard 
wood or zinc plates. One type of the machines which have now 
almost entirely replaced this primitive contrivance is shown in 
fig. 77, but is principaUy used for offal and common classes of 
goods. For better work, traversing rollers, such as Wilson's 
ingenious double-bed roller shown in fig. 78, are to be preferred. 
After rolling, the goods are dried pretty rapidly by the aid 
of moderate heat, and, after polishing with a brush (hand, or 
machine, -fig. 79), are ready for sale. It may be pointed out that 
although the tools are different, the process is almost the same 
as that used for " vache lissee " in France and Belgium, and 
closely resembles that of currying harness leather, except that 
the " stuffing " with fats and oil is omitted. 



VEGETABLE TANNING PROCESSES 367 

In contrast with the rather elaborate method just described 
we may place the American finish of red hemlock sides, which 




Fig. 78. — Wilson's Double-bed Butt Roller^ 




7— . ^^-F=:-=^^^ r\ 

Fig. 79. — Brushing Machine. 



are tanned throughout with a material which yields no bloom. 
On these, the scouring and " striking " is altogether omitted : 
the goods are completely dried out from the pits, which is found 



368 PRINCIPLES OF LEATHER MANUFACTURE 

to fix the dark-coloured liquor, and result in better colour ; they 
are then damped back, and tempered, and heavily rolled under 
a rapidly moving pendulum roller, which polishes at the same 
time that it smooths the leather. The saving of cost by so simple 
a process is not inconsiderable, and somewhat similar methods 
are gradually being adopted in this country, rolling taking the 
place of striking. 

In the West of England much heavy leather has been manu- 
factured from South American hides tanned with a large pro- 
portion of valonia, and which consequently %re heavily bloomed. 
No attempt is made to remove this bloom, which would too 
much lessen the weight and firmness, but the goods, after a light 
oiling to preserve the colour, are hung up and partially dried, 
and are then laid in pile to temper. The grain side is now wet 
with soap and water, with which a little oil is often mixed, and 
the bloom is " struck in " with the pin or machine — a somewhat 
blunt pin being used, or a blunt tool in the striking machine — 
which is held at such an angle as to smooth and compress the 
grain without taking too much hold on it. After a little further 
drying the striking is generally repeated, the goods are washed 
over with water, and rolled " on." They are now coloured with 
a mixture of pigment colour, generally containing a large pro- 
portion of whitening, or sometimes of French chalk coloured 
with ochres, chrome-yellow, and orange, or whatever may suit 
the tint preferred by the tanner, or best imitate the colour of a 
clean-scoured tannage, and usually mixed with size and oil, or 
sometimes with oil and tan liquor. This mixture is well rubbed 
in, and smoothed over with a cloth, and then polished by brush- 
ing, when the goods are " rolled off," rapidly dried, and again 
brushed. If the work has been well done,- it is not easy to dis- 
tinguish from clean scouring, and is much cheaper. 

A method intermediate between this and the first described, 
and which was formerly much used in London, was to proceed 
as above, but using more water and holding the pin in the first 
striking so as to scour out as much bloom as possible, and assist- 
ing this by the free use of water and the brush. Instead of 
using an opaque pigment-colour the goods were generally 
coloured either between striking and the first rolling, or between 
the two rollings, with a transparent colour, such as dissolved 
annatto, or a mixture of aniline dyes, so as to conceal the traces 
of bloom, and to render slight damages to the grain less con- 
spicuous. 

The process of sole leather tanning has been discussed in con- 
siderable detail on account of its simplicity and importance. It 



VEGETABLE TANNING PROCESSES 369 

is now time to point out in what respects the tannage of the 
Hghter leathers differs from it in principle. Taking the case of 
ordinary dressing leathers, such as kips and shaved hides, the first 
point to remember is that these goods come into the liquors not 
merely almost entirely deprived of lime by bating, but in a very 
flat and fallen condition from the action of the bacterial ferments 
of the bate. As a general rule in this country the colouring is 
done in paddles, but where a very smooth grain is required the 
use of suspenders is to be recommended, and in America is 
largely adopted. Indeed in the States the entire tannage of much 
of the cheaper leather is done in suspension, and the sides are 
only removed from the laths to which they have been nailed 
when they are required for splitting. It is obvious, from what 
has been said of sole leather, that as the hides are brought into 
liquors in a very fallen and extended condition, the grain will be 
likely to be wrinkled ; and indeed this is sure to be the case 
unless, by suspension, the hide is more or less kept in tension till 
its fibres are fixed by tanning. The free motion in the paddle 
favours the formation of a " pebbled " grain, since the hide is 
bent now this way now that, and minute wrinkles and creases are 
formed in all directions. For many purposes, and especially if 
a grain is afterwards to be raised by " boarding " the curried 
leather, this graining in the paddle is not disadvantageous so 
long as it is not excessive. In some other cases it causes much 
trouble and labour to the currier before it is removed, and if 
the English tanner and currier are ever to compete with the 
American in smooth grain finishes, it will be necessary for them 
to obviate this source of wasted labour. The graining is the less 
considerable, and the easier to remove, the weaker and more 
mellow are the liquors employed in colouring and the more 
gradually their strength is increased. 

The production of a soft leather depends on the fibre being 
tanned in a fallen and unswelled condition. It is for this reason, 
and to remove the elastin-network from the grain-layer, 
that bating is in many cases essential, though where somewhat 
firmer leathers are required, mere reduction of the swelling by 
removal of the lime is sufficient. For the same reason no acid- 
swelling is permissible either before tanning or in the liquors, and 
though liquors for soft leathers must be faintly acid, they are 
incapable of removing any large quantity of lime, and for the 
best results the deliming must be complete before tanning. 

Now that in the light of knowledge very recently acquired we 
have much clearer ideas than formerly of the exact objects of 
bating and puering, it ought in many cases to be possible to 

24 



370 PRINCIPLES OF LEATHER MANUFACTURE 

dispense with these operations altogether in favour of a scientific 
system of dehming and reducing the alkaline swelling. At a 
certain definite degree of acidity, the " isoelectric point," which 
for gelatin is about F^=4-y, and for hide is almost identical, as 
Porter has recently shown (see footnote, p. 578), the hide- 
fibre is in a neutral condition, acting neither as an acid nor an 
alkali, and at the same time is in the least swollen condition 
possible by merely chemical neutralisation. It would be im- 
possible to maintain this condition by the mere addition of any 
tolerably strong acid, but, taking advantage of the properties of 
weak acids and of their neutral salts (p. 99), it can be done, 
and would be very approximately attained by mixtures of 
acetic acid and sodium acetate, the Ph of which . is little varied 
from 4-7 by small additions of acid or alkali, but it is very 
possible that better and cheaper solutions may be devised. 
The concentration would be regulated and maintained by the 
addition of smal] quantities of acid or of sodium acetate to the 
neutral point of alizarin red, which closely corresponds to the 
isoelectric point {cp. p. 201), using the comparator if the liquor 
is coloured. Such regulating salts are usually called " buffers." 

Another result of puering is the digestion of the elastin of 
the grain and of any remnants of the glands and hair-follicles 
which remain, and this would be done if necessary by pancreol 
or some other tryptic ferment, in mixture with ammonium 
chloride, and preferably before the neutralisation, as these act 
most favourably in a somewhat more alkaline solution, indicated 
approximately by the neutral point of cresol red. The ammonium 
chloride itself acts as a buffer. 

As mere bating or puering is mainly designed to reduce swelling 
by the action of bacterial products (p. 218), and is not a very 
efficient means of removing lime, it is desirable where it is em- 
ployed, to supplement it by some more active deliming process. 
In the lighter leathers drenching (p. 214) generally fulfils this 
purpose, and many of the more intelligent tanners now give bated 
hides a bath in boric acid before tanning, which not only removes 
the last traces of lime without acid-swelling, but checks' the 
bacterial fermentation, and prevents its introduction into the 
liquors. In gambler tannages, a decidedly better colour is 
obtained by this treatment (p. 362). 

In most cases the production of bloom is not desired in dressing 
leather tannage, and is prevented by relying chiefly on hquors, 
and avoiding the use of bloom-giving solid materials, which include 
most pyrogallol tannins. Dressing leather tannages .can fre- 
quently be advantageously hastened by drumming, which by 



VEGETABLE TANNING PROCESSES 371 

continuously bending the leather in all directions constantly 
widens and contracts the spaces between the different fibres, and, 
as it were, pumps the liquor through the skin, but of course 
tends to produce a " pebbled grain." The softness of dressing 
leathers is increased, and the hardening action of acids present 
in the liquors is prevented by the addition of salt, or of some 
sulphates (sodium, magnesium, ammonium) which exercise a 
sort of pickling action on the fibre and prevent its swelling, but 
at the same time tend to light weight and a somewhat empty 
tannage. It by no means follows that a hide or skin which is 
thoroughly coloured through is really fully tanned ; as, though 
the fibres may be actually tanned or coated on the surface, time 
is required for the tannins to penetrate them to the centre. This 
incompleteness of saturation is often found in drum-tannages. 
Such leathers are generally tough, and gain weight and softness 
in currying. In order to " carry grease " well, that is, to absorb 
a large quantity without appearing greasy, it is essential that the 
fibre bundles should be thoroughly split up or differentiated ; and 
the degree to which this is attained largely depends on the extent 
of liming. There is also considerable difference in different 
tannages as to the amount of grease which they will carry. 

It is now not uncommon to combine a degree of alum or 
chrome tannage with vegetable tannage in the finer dressing 
leathers. For further information on this the reader must be 
referred to the next chapter. 

The finest sorts of leather, such as goat, calf, sheep, and seal, 
for bookbinding, upholstery, and the like, are mostly tanned with 
sumach, paddles and drums being largely used to quicken the 
operation. Leather tanned with sumach has been proved by the 
researches of the committee of the Society of Arts on the decay of 
bookbinding leathers ^ to be the most durable leather for this 
purpose, some other tanning materials of the pjnrogallol class 
coming near it in this respect, while all catechol tannages are 
found peculiarly liable to destruction by the action of sunlight, 
dry heat, gas fumes, and traces of sulphuric acid from other 
sources, although in many cases more durable than the pyrogallol 
tannages when merely exposed to riiechanical wear and moisture, 
as is the case with shoe leather. East India sheep- and 
goat -skins, so-caUed " Persians,' are tanned with the catechol 
tannin of Cassia auriculata (turwar, tarwad, or avaram) bark, 
and are very easily affected by light, so much so that a photo- 
graph can be printed on them in sunlight from a negative (p. 329) . 

The finer leathers of which we have just spoken are almost 
^ Soc. of Arts Journ., 1901, p. 14. 



372 PRINCIPLES OF LEATHER MANUFACTURE 

invariably prepared for tanning by puering and drenching, as 
colour and softness are the principal characteristics aimed at. A 
somewhat interesting style of tannage is occasionally used for 
sheep-skins (roans) and calf-skins, in which the skin is sewn into 
a bag, flesh side out, with only a small aperture left for filling in 
one of the shanks. It is then turned grain, side out and filled 
with strong sumach Hquor and a little leaf sumach to prevent 
leakage, and floated in a tank of warm sumach hquor. After a 
short immersion the skins^are piled on a stage, so that the 
liquor is pressed through them by their weight, and when partially 
empty they are refilled, and the process repeated. The tannage 
is complete in about twenty-four hours, and the leather is very 
soft. 

Although the various chemical and physical theories of the 
tanning process are discussed at some length in Chapter XXXII. , 
it may not be out of place here to point out certain chemical 
facts which are not in dispute, and of which the knowledge may 
help to make clear the reasons governing the methods of manu- 
facture which have just been described. In the preparatory 
processes through which the skin has passed, whether for sole or 
dressing leather, the conditions have been prevaihngly alkahne, 
and even if acid deliming is practised, the aim is rather to bring 
the skin to a neutral than an acid condition. For the actual 
tannage, however, it is essential that the reaction should be acid, 
though the actual degree of acidity as measured by the hydrion 
concentration will vary much with the class of leather produced, 
being somewhat considerable in the case of sole leather, and but 
httle on the acid side for the finer and softer leathers. Whatever 
the nature of the combination of tannin and pelt it only takes 
place in acid solutions, and though it is easy to bring the diffi- 
cultly soluble tannins, such as the " reds " of quebracho, into 
alkaline solution, they fail to tan in that condition, and recourse 
must be had to such salts as bisulphites, which, while they have 
an acid reaction, are so weakly acid that their bases (soda in the 
case mentioned) can form soluble salts with the weak acids of 
the difficultly soluble tans. As to the reasons for this peculiarity 
Chapter X. must be consulted, but it may be pointed out here 
that alkaline liquids not only do not tan, but actually " strip " 
the tan already deposited, and this, is even true to some extent 
of bisulphites. Apart from this effect on the tannins, the acids, 
especially in sole leather tannage, have of course a direct effect 
on the skin itself, swelling and separating the fibre bundles of 
the corium into their constituent fibrils, and so presenting a 
larger surface to the action of the tan, and facilitating its pene- 



VEGETABLE TANNING PROCESSES 373 

tration. In the earlier stages of the tanning process the untanned 
and freely exposed fibres combine eagerly with the tannin, and 
while weak liquors are sufficient to supply this, very large 
volume and rapid change of liquor is required to properly 
feed the skin. A large proportion of the total tan is absorbed 
in the first weeks of the sole J'eather tannage, and of course 
in still shorter time by the thinner leathers, and this em- 
phasises the importance of rapid and continuous feeding in this 
stage. There is no reason to doubt that the tannin combines 
instantly with the raw gelatinous fibre when it comes in contact 
with it, but as the tannage progresses and the outside fibres are 
saturated, the liquor can only get at the fibres within by a slow 
process of diffusion, which is probably rendered still slower by 
the tanned surface acting as a more or less "semi-permeable 
layer." Now, diffusion can only take place from a stronger to a 
weaker solution, and hence the necessity of continuously increas- 
ing the strength of the liquor so that it should always be stronger 
than that contained in the tanned outer layer, aftd this continues 
to be true throughout the process. If the barkometer-strength 
of a liquor be taken at intervals, it will be found to diminish 
rapidly at first, then more slowly, and finally to remain almost 
constant, while the tannage at the same time comes to a stand- 
still. One of the reasons, beside the greater strength of the 
liquors, why modern tannages are much shorter than they used 
to be is, that more care is taken that the liquors are changed 
before they have ceased to act, and there is no doubt that in the 
old two-years' tannage of sole leather much time was wasted in 
long layers, in the latter part of which little or no progress was 
made. Most rapid-tanning processes depend on the quickening 
of diffusion by mechanical means. Thus the drum-tannages, by 
constant flexure of the hide, alternately compress one side or 
the other, and thus produce a sort of pumping action, which 
forces the liquor through; in "bag-tannage" and some similar 
processes the liquor is forced through by direct mechanical 
pressure ; while in the Nance High Vacuum process ^ the liquor, 
though only at a temperature of 70° to 80° Fahr., actually boils 
in the interior of the hide, and so produces rapid interchange. 
In the earlier stages of tanning the liquors penetrate the 

1 The attempt to force liquor into hide by direct pressure or moderate 
vacua, as is done in the creosoting of timber, has always resulted in failure^ 
as the pores of hide are filled with practically incompressible water, while 
those of timber contain air. If the partially tanned hides were dried 
before impregnation, liquor could easily be forced through them in this 
way. 



374 PRINCIPLES OF LEATHER MANUFACTURE 

fibrils and combine with them (whether chemically or by 
" adsorption "), and render them incapable of swelling in water 
or of putrescence, and actually converted on the surface into 
leather/ but such leather is porous and hght-weighing, and 
quite unsuitable for practical use, especially as sole or belting 
leather. 

As the tannage progresses the fibres may be assumed to be 
completely converted into leather, but they are still capable of 
fixing tannins, and especially the less soluble tannins, and such 
bodies as ellagic acid, by adsorption or crystallisation, and the 
leather gains in weight and in firmness, but this part of the 
process must be very limited in dressing leathers. The case is 
very parallel to that in the chrome process, where, after the first 
stages, only the more basic and less soluble salts are fixed. After 
the completion of the tannage in the yard it is very customary 
to " vat " the leather in the case of sole leather in strong and 
warm solutions of highly sulphited extracts, but this must rather 
be considered a bleaching than a tanning operation, as its effect 
is to remove a part of the phlobaphenes and difficultly soluble 
tannins which have been deposited, and to render the leather 
softer and more porous and permeable to water. In dressing 
leather yards where mixed tannages are used it is customary, 
with a similar object, to drum the goods in a warm sumach or 
myrobalans liquor, but this with the lighter leathers is more 
defensible. 

Even when the goods are removed to the shed the tannage 
can hardly be said to be quite complete, as no doubt further 
fixation of the soluble tannins goes on during the drying, and 
some of the non-tanning matters, such as gallic acid, undergo 
oxidation and dehydration, and exert a tanning effect if the 
fibre is not already fully saturated {cp. p. 574). 

1 In this stage the microscope shows no signs of a raere coating of the 
fibres such as Knapp supposed. 



CHAPTER XXII 

COMBINATION OF VEGETABLE AND MINERAL TANNAGE 

In very early times leathers were produced which were partly 
tanned with alum and partly with vegetable materials. One 
of the earliest of these was probably the Swedish or Danish glove- 
leather. The principle has long been applied to the production 
of certain very tough and flexible leathers known as " green 
leather," and used for " picker -bands " for looms, laces for belting, 
" combing-leathers," and some other purposes where softness and 
toughness are of principal importance. It was applied in America 
by James Kent to the manufacture of an imitation of glazed kid, 
which he named Dongola leather ; and since that time the 
method, in various modifications, has taken a considerable place 
in the manufacture of the finer leathers for shoe purposes, 
especially in the United States. 

Alum-tanned leathers, as has been already stated, are remark- 
able for softness and toughness, and the mineral (crystalloid) 
tannages have the power of penetrating and isolating the indi- 
vidual fibrils of the skin in a much greater degree than the vege- 
table tannins, and hence are less dependent than the latter on 
a previous isolation produced by liming or swelling. On the 
other hand, they give much less plumpness and solidity and 
more liability to stretch, and, as a general rule (to which chrome- 
tannages are an exception), are less resistant to the action of 
water, and incapable of producing a soft leather without 
mechanical softening (staking) after the tannage is completed. 
Purely mineral tannages have always a woolly fibrous structure, 
and never the firm and compact flesh which is required in leathers 
which are to be " waxed " or finished on the flesh side to a 
smooth surface, and as they communicate more or less of these 
peculiarities to combination-tannages, the latter are mostly used, 
either for grain-finish, or for uses where a soft and velvety flesh 
side is required, as in the case of " ooze " or " velvet " calf. On 
the other hand, the partial use of vegetable tannage communi- 
cates to them a degree of plumpness, fulness, and resistance to 
water which is not possible to alum-tannages pure and simple, 
and a softness which is not easily obtained in vegetable tannage 
without the use of large quantities of fats or oils. A preliminary 

375 



376 PRINCIPLES OF LEATHER MANUFACTURE 

mineral tannage also greatly increases the rapidity of the pene- 
tration of the vegetable tans, by isolating the fibres, and rendering 
them less gelatinous. Once a leather is thoroughly tanned by 
vegetable materials, it is little affected by subsequent treatment 
with alumina, or even with chrome ; and, on the other hand, 
though chrome and alumina leathers are still capable of absorbing 
considerable quantities of vegetable tannins, they always retain, 
in a degree, the qualities which the mineral tannage has com- 
municated to them. The resulting leathers are thus not only 
modified by the different proportion of vegetable and mineral 
tannages which have been given, and by the properties of the 
particular vegetable tannage used, but by the order in which 
the several treatments have been given, and always retain, to a 
considerable extent, the characteristics of that which has been 
first applied. We have thus in our hands a powerful means of 
modifying the character of our leather to suit the special require- 
ments which it is to fulfil. 

So long as tanners were restricted, on the one hand, to the 
ordinary methods of stuffing tanned leathers with oils and fats, 
and on the other to the use of egg-yolk, which had long been 
common in alum-tannages, combination-tannage remained of 
but secondary importance, and it was the application of the 
method of " fat -liquoring " by James Kent to his Dongola^ 
leather which gave them the place they now possess, by providing 
a cheap substitute for egg-yolk, and enabling the tanner to obtain 
softness and resistance to water without producing the greasy 
feel which is common to curried leathers. The process of fat- 
liquoring has already been mentioned in connection with chrome 
leathers, to which it was subsequently applied, and we shall 
return to it after having given some further details of the methods 
of tannage. 

In the first place, we must consider briefly the mutual action of 
the mineral and vegetable tannages on each other. It has been 
pointed out by Eitner, and also mentioned in connection with 
the decoloration of extracts (p. 405), that the addition of | per 
cent, of alum or aluminium sulphate to tanning liquors lightened 
their colour, not only by giving a degree of acidity to the solution, 
but by precipitating a portion of the darker and less soluble 
constituents, and made the solutions yellower by developing the 
colour of the flavone mordant dyestuffs which are contained in 
most tanning materials. It is therefore desirable, if these salts 
are used, to allow the solution to subside, or to filter off the dark- 

1 " Dongola " leather was invented in the time of the Egyptian War, 
when Dongola was much in the public mind, hence its fancy-name. 



VEGETABLE AND MINERAL TANNAGE 377 

coloured precipitate. Chrome and iron salts no doubt produce 
a similar effect, though from the dark colour of their compounds 
with tannins the lightening of colour does not take place. 
Potassium dichromate, especially if acidified, generally precipi- 
tates, oxidises, and darkens tannins, so that it is not practicable in 
combination-tannage (pp. 495, 506) to follow a vegetable by a two- 
bath chrome tannage, though the reverse order may be pursued. 

Combination-tannages, such as the Swedish and Danish glove- 
leathers already referred to, are generally first tawed with alum 
and salt, with or without flour and egg-yolk, and are then coloured 
and more or less tanned with vegetable materials. That em- 
ployed on the original Danish leather was willow bark (of Salix 
arenaria). In France, where this willow is not found, the bark 
of the commoner Salix caprcBa was substituted, and as it is 
much weaker in tannin, additions of oak-bark or sumach to supply 
the deficiency, and of madder to give a redder colour, were made 
to it. The dyeing of these leathers is frequently combined with 
the tannage, dyewoods or dyewood liquors being mixed with the 
tanning liquors. In the manufacture of glazed French kid, indeed, 
the process is so arranged, by brushing on dye-liquors mixed with 
tannins, as merely to tan the grain-surface, which is necessary to 
enable an alumised leather to be glazed by friction, leaving the 
substance of the leather of purely alum tannage. 

On the other hand, in the " green leathers " (so called from 
their greenish-yellow colour, arid largely made in the West 
Riding of Yorkshire) the hides usually receive a light gambler 
tannage, extending over a week or so in weak gambler liquors 
in handlers, and are then " cured " by handling in hot and strong 
solution of salt and alum, in which they are finally left all night, 
and then dried rapidly without washing out the alum, much of 
which consequently crystallises on the surface. This is slicked 
off, and the leather damped back, and heavily stuffed with sod- 
oil. If, however, the combination-tannage is properly carried 
out, it will stand liberal washing without losing the necessary 
alum, and of course a tougher and more satisfactory, though 
somewhat lighter-weighing, leather results. It is in many- cases a 
better plan to combine the two tannages in one bath, mixing the 
alum and salt with the gambler, and handling or paddling the 
goods in the mixture. This is the plan usually adopted for 
Dongola leather in the United States. For skins which are to 
be glazed it is important that the surface should be tanned with 
the vegetable material, and the goods are therefore worked into 
gambler liquors, to which the salt and alum are only added after 
the tannage has made some little progress; while for dull Dongola, 



378 PRINCIPLES OF LEATHER MANUFACTURE 

intended rather to imitate calf-kid, it is best for the alum and 
salt tannage to begin first. For goat -skins for glazed Dongola 
kid about 4 lb. of block gambler, J lb. of alum, and J lb. of salt 
are used per dozen, and the tannage occupies in all about twenty- 
four hours. 

After the skins are tanned they are thoroughly washed out 
with tepid water to remove loose alum and gambler, and are then 
ready for fat -liquoring. As in the case of chrome leather, it is 
of great importance that this washing should be done thoroughly, 
as any remaining alum which diffuses into the fat-liquor will 
cause it to curdle. A treatment with a weak neutralising solution, 
say of hyposulphite, in the first washing would do no harm. If 
the washing is thorough, the more neutral the fat-liquor the 
better, but a more alkaline solution is less liable to curdle, and 
the original fat -liquor used by Mr Kent was the very alkaline 
liquor which had been used for washing chamois leather, and 
therefore contained a good deal of degras. Now, soap and oil 
solutions are usually made for the purpose, and those described 
in the chapter on chrome tannage are quite suitable, though fat- 
liquoring is somewhat easier than in the case of chrome, and the 
better the oil is emulsified, the more satisfactory is the result. 
A sheet-metal cylinder with a piston covered with fine wire- 
gauze does good service as an emulsitier, and another method is 
to incorporate the oil thoroughly with a hot pasty solution of 
the soap, which can be diluted as required. If special soaps are 
made by the cold process (p. 427), they can be superfatted in 
making as required. Oils containing a little free oleic acid 
emulsify most easily, and sulphonated oils are also suitable. 
For glazed finishes it is a very common mistake to employ too 
strong a fat -liquor — even | per cent, of soap and | per cent, of 
oil will produce a very noticeable softening effect — but of course 
for dull finishes much more may be used. Leathers absorb the 
fat -liquor most readily in a sammed condition, but even if quite 
wet they soon take up the whole of the oil and soap, leaving only 
a little turbid liquor in the drum. 

Not only combination-tannages but purely vegetable ones can 
be fat -liquored, and the process is now largely used for coloured 
calf and other leathers which are required to be soft and nourished 
without greasiness. Sesame (Gingelly) oil seems very suitable 
for this purpose. East India sheep and goat, though they do 
not generally appear greasy, are often so heavily oiled with this 
oil (up to 30 per cent, of their weight) that it is usually desirable 
rather to diminish than increase it, which may be done by wash- 
ing with soap-solutions, preferably before aluming. Goods may 



VEGETABLE AND MINERAL TANNAGE 379 

be blacked while still wet with fat -liquor, but, except in the case 
of chrome combinations, should generally be dried out before 
dyeing, as this fixes the oil and soap on the fibre. 

Many leathers are now made by being coloured and partially 
tanned with ordinary vegetable materials, and finished with a 
Dongola tannage with alum, salt, and gambler, and fat -liquoring. 
In the United States the tannage is often begun by suspension 
in hemlock bark liquors. 

Imitations of Dongola leather can be made by treating East 
India sheep or goat, or other lightly tanned stock, with an alkaline 
solution, such as dilute borax, ammonia, or soda, similar to those 
used in neutralising chrome leather, but somewhat stronger, to 
strip a portion of the tan, and then retanning with alum and 
salt, fat -liquoring and finishing in the Dongola manner. In 
place of mere alum and salt a " neutralised " basic alum solution 
is used with advantage (p. 242). 

" Semi-chrome." An exactly similar process may be used, em- 
ploying instead of alum a basic chrome liquor such as is used in 
the one-bath chrome process, and the substitution may be so 
complete that the finished goods are difficult to distinguish from 
full chrome, and may even stand a moderate boiling test ; and 
during the war this process was used by many tanners who 
thought it easier than " straight " chrome. 

Chrome combination may also be made by retanning the 
chromed leather produced either by the one- or the two-bath 
process with vegetable materials, of which gambler or sumach 
seem the most suitable; but considerable care is needed, as re- 
tannage, even with very weak liquors, deprives chrome leather 
of its stretch, and if carried to excess readily makes it hard and 
tender. It may, however, be useful for some mechanical leathers, 
where the stretch is a disadvantage, and for treating chrome splits 
so as to make it possible to " wax " them. Chrome leathers 
cannot be "stripped" by alkaline solutions, but may be by 
a solution of Rochelle salt (p. 572), or by oxalic acid (Lamb's 
patent, p. 571). 

Various other sorts of combination are possible beside those of 
mineral and vegetable tannage, as, for instance, of formaldehyde 
with both mineral and vegetable tannages, of which many forms 
have been tried, between this and peat or humus tannage (Payne), 
the halogen tannages mentioned on p. 575, quinone tannage, 
and many others, but none of them have reached practical 
importance. Chrome and iron combination tannages are noted 
on p. 289. 



CHAPTER XXIII 

GRINDING OF TANNING MATERIALS 

Before the tannin they contain can be extracted, most materials 
require to be ground, almost the only exceptions to this rule 
being divi-divi and algarobilla, in which the tannin is very 
loosely contained. Extracts, whether solid or liquid, merely 
require to be dissolved in water or liquor, in which they are, 
for all practical purposes, perfectly soluble. With the less 
soluble extracts it is generally preferable to dissolve at a tempera- 
ture of 50° to 60° C. with vigorous stirring. For solid extracts 
some method of mechanical agitation is desirable. 

The actual method of grinding, and consequently the machinery 
employed for the purpose, vary not only with the material to be 
ground, but with the method of leaching adopted, as it is essential 
that the mass of ground material should be completely permeated 
by the liquor employed in leaching ; and if it be ground too 
finely, or subjected to too much pressure on account of the height 
to which it is piled in the leaches, it is apt to form a compact 
and clay-like mass, the interior of which remains unextracted. 
This is specially important in the " press " or circulating systems 
now generally adopted. 

In the laboratory, where thorough extraction must be com- 
pleted in a few hours, the material can hardly be too fine ; but 
on the larger scale a much coarser product must be used, and 
leaching requires days, or sometimes even weeks, and is then 
seldom successful in removing all the tannin. It is probable, 
however, that in the future these mechanical difficulties of ex- 
traction will be overcome ; and the material will then be as finely 
divided, and as completely extracted on the large scale, as it is 
in the laboratory at the present time. 

One of the earliest methods of grinding oak-bark, and which 
is still used for sumach (p. 308), consists in crushing it under 
large circular edge-stones, frequently turned by a horse. This 
process was very slow and inefficient for barks, and both it and 
horizontal millstones similar to those used for wheat were long 
ago superseded by iron or steel mills on th^ same principle as the 
ordinary coffee-mill. 

These mills (fig. 80) consist of a " bell " or inner cone, covered 

.380 



GRINDING OF TANNING MATERIALS 381 




with blades or teeth arranged at a shght angle to the vertical 
section of the cone, and which are made finer and increased in 
number towards its lower and wider part. This cone rotates 
within an outer hollow cone or casing, also provided with blades 
or teeth which are sloped slightly in the opposite direction to 
those of the inner cone, so as to meet them at an angle, like 
the cutting-blades of a pair of scissors, and the angles of the cone 
are so chosen that the blades ap- 
proach each other more closely to- 
wards their base. The outer cone 
is fixed, and is provided with a 
hopper like a coffee-mill, while the 
inner cone is so rotated on its axis 
that bark placed in the hopper is 
screwed down between the two, and 
cut finer and finer till it reaches the 
lower edge, when it drops out. The 
blades or teeth are usually cast in 
one piece with the metal cones, and 
sharpened when required by chipping 
with cold chisels. This operation 
should not be conducted in the 
mill-house, or small chippings of 
iron may get mixed with the bark 

and cause stains on the leather. This form of mill, which is 
run in England at about thirty revolutions per minute, and 
nearly three times as fast in America, works very well with 
dry material, but clogs badly if it be appreciably damp. On 
this accoant it is always well to run the mill with a fairly slack 
belt which will slip before exerting sufficient pressure to break the 
machine, as in such operations as grinding, safety-clutches are 
of but little use. 

A type of mill varying somewhat from the above consists of 
a pair of discs or very obtuse cones, the inner one of which runs 
on a horizontal axis. The teeth are generally arranged in con- 
centric rings and interlock with each other, so that the opera- 
tion of these mills is a direct " shearing " and not a clipping one. 
The material to be ground is fed at or near the centre of the fixed 
disc, and escapes at the edges. The construction of this class of 
mill will be easily understood from fig. 81. With damp materials 
the discs very readily become clogged between the teeth, and 
small pieces of iron or steel which get caught between the teeth 
will often result in the breaking of the latter and the formation 
of iron dust, which is a serious objection to the employment of 



Fig. 80.— Cone Mill. 



382 PRINCIPLES OF LEATHER MANUFACTURE 



this type of mill (to which the Schmeija " Excelsior," the Glaeser 
" Favorita," and the " Devil Disintegrator " of the Hardy Patent 
Pick Co. belong) for grinding barks. 

Myrobalans and mimosa barks have proved especially trouble- 
some to grind, the former from the hardness of the stones of the 
fruit and a tendency to clog the mill, and the latter from their 
combined hardness and toughness. " Disintegrators " of various 
patterns are now made which are capable of grinding both 







Fig. 8i. — " Excelsior " Mill. 

these materials satisfactorily, and, but for their liability to cause 
fire, and the larger proportion of fine dust which they make, are 
usually to be preferred to toothed mills. For these reasons the 
mill-house should as far as possible be isolated from other parts 
of the tannery. In spite of their disadvantages, however, they 
have come very largely into use on account of their efficiency 
in grinding obstinate materials. Disintegrators work on the 
principle of knocking or beating the material to powder by 
means of very rapidly revolving beaters, which, in the smaller 
machines, are driven at 2500 to 3000 revolutions per minute. 

The first disintegrator was made by Carr, and consisted of 
two concentric cyHnders or baskets of steel bars rotating in 
opposite directions at a very high speed. The material was fed 
between these, and was dashed to pieces by being thrown against 
the bars and the outer casing. 



GRINDING OF TANNING MATERIALS 383 

A simpler form was soon introduced by Carter, in which only 
one axis was employed, carrjnng radial beaters which dashed the 
material against the serrated outer casing, a portion of the cir- 
cumference of which was fitted with gratings, through which the 
ground material was thrown as soon as it was sufficiently reduced 
in size, the fineness of the grinding being regulated by changing 
the grates as required. This type of disintegrator is, with slight 
variations, made by all the leading makers of tanners' machinery ; 




Fig. 82. — Disintegrator. 



and one form is shown in fig. 82, and a similar but smaller 
machine, opened to show construction, in fig. 83. 

In the more modern machines the sides as well as the cir- 
cumference of the casing are frequently corrugated in order to 
increase the action on the material. 

Mills running at such high rates of speed as 3000 revolutions 
per minute will grind most hard substances, such as stone or 
brick, without injury, but pieces of iron among the tanning 
material are apt to cause damage, and various magnetic devices 
have been employed for separating this metal, but with only 
partial success. In the best mills, therefore, the beaters and 
inner casings are constructed so that they can be easily replaced, 
and the damage is then rarely serious. 

In order to avoid vibration the discs and beaters of all these 
high-speed mills must be balanced with great accuracy. This is 
best accomplished by removing the spindle from the mill and 



384 PRINCIPLES OF LEATHER MANUFACTURE 

allowing it to roll on two levelled straight-edges, and then filing 
or chipping the beaters on the heavy side until it will remain 
indifferently in any position. 

A form of disintegrator has been brought out in America by 
the WiUiams' Patent Crusher and Pulveriser Company, in which 




Fig. 83. — Disintegrator opened, showing construction. 



a series of discs are keyed to the main shaft, to the circumference 
of which a number of sets of " hammers " are suspended by 
means of hinge-bolts. Each of these steel bars, or hammers, 
has a free arc movement of 120°, and when the machine is in 
motion take a position divergent from the centre on account of 
the centrifugal force. After striking a blow against any material 
fed on to a plate serving as an " anvil " the hammers recoil, 
and, after passing any material which is not shattered by the 
blow, again resume their normal position, leaving the next set 
of hammers to beat against the unground material. The hinged 
suspension of the hammers imparts a degree of flexibility to the 
mill which is not found in any other machine of this character, 



GRINDING OF TANNING MATERIALS 385 

and lessens the risk of serious damage to the machine by the 
introduction of pieces of metal along with the bark. The makers 
claim that this machine can be repaired more rapidly and with 
less expense than any other disintegrator of equal power on the 
market. Considerable improvements have recently been made 
in the details of its construction. Fig. 84 shows a section of this 
mill. Of course only the end hammers of each set can be seen 



1901 PATTERK 




Fig. 84. — Section of Williams' Crusher. 



in the figure, and several improvements in detail have now been 
made. 

It is necessary that the feeding aperture of disintegrators 
should be well protected to prevent the escape of fragments. 
The writer remembers the case of a girl who lost an eye through 
an escaping fragment of glue which was being ground. 

When myrobalans or valonia is to be used for leaching, it is 
generally better to crush it between toothed or fluted rollers 
rather than to grind it finely, as the cellular structure is just 
as completely broken up, and the flakes formed by crushing allow 
of much freer percolation than when the material is powdered 
by the disintegrator, while the consumption of power is also less. 
The general construction of the machine will be easily understood 
from fig. 85, and it is only necessary to point out that the small 
upper roller acts mainly as a " feed " to the larger crushing rolls. 

25 



386 PRINCIPLES OF LEATHER MANUFACTURE 

In the best mills the rollers are made up of a series of toothed 
steel discs on a square axis, and are on this account easily re- 
placed or sharpened when they have become broken or worn. 

Several mills have been introduced in America in which the 
bark is sawn or rasped by toothed discs like circular saws, but 
these are only capable of dealing with barks of a brittle nature, 
and are immediately choked by tough materials like the bark of 
the mimosa or oak. A better form of mill, but one which is, to 
some extent, subject to the same disadvantage, is the " shaving- 
mill," in which blades are fixed like plane-irons upon a disc, 




Fig. -^5. — Myrobalans Crusher. 

cones, or cylinder, and are rotated at a high speed against the 
material, which is fed against them by' toothed rollers at such an 
angle that the shavings are cut diagonally to the grain. These 
shaving-mills are largely in use in America for hemlock bark, 
with which they are particularly successful. The principle of 
the machine is exactly the same as that of the machines used in 
cutting oak-wood, quebracho, and the different dyewoods. One 
type of shaving-mill is iUustrated in fig. 86. Such machines are 
only suited for cutting rather thick bark in " long rind," and are 
quite unsuitable for chopped bark. 

It frequently happens that the material is delivered from the 
mill in a very unequal state of division, and it is sometimes 
necessary to screen it and thus separate the coarser portion either 
for use in the leaches or for re-grinding, while the finer portion 
is more suitable for " dusting." With disintegrators, which 
deliver the bark with considerable impetus, the screening can be 
accomplished by placing a screen diagonally below the mill, 
through which the finer parts are projected. It is, however, 
essential that this screen should be quite smooth on its upper 



GRINDING OF TANNING MATERIALS 387 



surface and very strong, as ordinary wire gauze is immediately 
cut through by the impact of the material. What are called 
" locked wire screens," in which the wires are supported by being 
actually twisted round the transverse bars, are very suitable. 
Where the circumstances will not permit of screening in this way, 




Fig. 86. — Shaving-mill. 



cylindrical rotating screens, or nearly horizontal screens vibrated 
by an eccentric, may be used. The latter are cheaper to erect, 
and have the advantage that they take up less room, and by 
having lengths of wirework or perforated steel of different coarse- 
ness the material may be separated into more than one degree 
of fineness. 

Oak-bark as it is taken off the trees is usually in lengths of 
perhaps 3 feet, and it is necessary to cut or break it into smaller 
fragments before it can be ground in most of the machines just 



388 PRINCIPLES OF LEATHER MANUFACTURE 




described. This is frequently done by hand by chopping the 
bark into pieces about 4 inches long, and the operation is known 
as "hatching." Machines on the principle of the chaff-cutter, 
consisting of a fly-wheel with curved blades radially attached to 
it, are sometimes used. Instead of " hatching "it, the bark is 
frequently broken by passing through toothed rollers fitting into 

each other, and often at- 
tached to the mill ; the 
construction of this machine 
will be readily understood 
from fig. 87. 

In Belgium, and some 
other bark-producing dis- 
tricts, the adhering moss 
and dead outside bark 
are usually removed before 
hatching, but apparently 
these impurities are fre- 
quently re-mixed with the 
bark after the hatching is 
completed ! As such barks 
often also contain much 
clay and dirt, it is gener- 
ally expedient to pass the 
hatched bark over a coarse screen before letting it enter the 
mill, so as to remove the greater part of such rubbish, since, if 
left in the bark, it produces black and unsatisfactory liquors. 

In drawing up policies for fire insurance it is usual to charge 
a higher rate where disintegrators are used to grind the tanning 
material, as, owing to the amount of dust and the production of 
sparks by the striking of the steel parts of the machine on any 
chance piece of flint or metal which may get into it, there is a 
greater liabihty to fire than with toothed mills, although with 
proper precautions the risk is really small {cp. p. 546). 

All disintegrators act like ventilating fans, and suck in air 
with the material, blowing it out again with great force at the 
periphery. This air is heavily laden with dust from the tanning 
material, which is extremely irritating to the lungs. The diffi- 
culty is to some extent remedied by an air-channel or flue 
(generally cast in the casing of the machine) connecting the dis- 
charge with the feed-opening so as to convey the air back to the 
disintegrator. The air is thus circulated through the arrange- 
ment, but some is always drawn in from the external atmosphere 
and driven out with the ground material, and it is advisable that 



Fig. 87. — Bark Breaker. 



GRINDING OF TANNING MATERIALS 389 

the chamber into which it is discharged should be provided with 
some means of filtering the air before it escapes. One convenient 
method is to have a large suspended flannel bag which is blown 
out by the air hke a balloon and out of which the dust can be 
shaken when the machinery has stopped. Another efficient 
method is to have one of the walls or the ceihng of the chamber 
made of canvas or of sacking ; but in any case the air should be 
allowed an escape where a little dust will not cause annoyance. 

Chain-conveyors. — ^While in England the ground material is 
usually carried from the mill to the leaches in barrows or baskets, 
in America the use of conveyors is practically universal, and 
there is no doubt that they effect a great saving of labour at a 
comparatively small cost. 

The most practical conveyor for tanning materials consists of 
a trough through which an endless chain passes carrying scrapers. 
The chain generally used for this purpose is one consisting of 
square links fitting into each other and capable of running over 
toothed wheels. These chains are made by several firms in 
America, and in England by the Ewart Chain Conveyor Co. of 
Derby, who supply not only plain hnks but also those having 
projections to which buckets, scrapers, and a variety of attach- 
ments may be fixed. 

In many cases the trough is V-shaped, with the chain running 
in the angle ; in others flat-bottomed, as in the illustration, or 
rectangular. The scrapers may consist either of metal or of 
wood ; and where materials have to be carried up a steep incline, 
buckets instead of scrapers should be employed. The arrange- 
ment of such a conveyor is illustrated by fig. 88. 

A useful form of conveyor for dry materials consists in a 
woven cotton belt running in a smooth trough and with laths 
riveted across it at intervals. These laths should project shghtly 
beyond the edges of the belt so as to prevent wear. Care must 
be taken with belts of this sort that the material does not get 
between the belt and the pulley. 

Chain-carriers are often used for conveying the spent tan to 
the furnaces from the leaches, and occasionally for carrying skins. 
Beside the malleable cast links which have been spoken of, 
and which are so arranged that they can be unhooked from 
each other when the chain is slack, various forms of malleable 
links are also available. 

Several other kinds of conveyor are in use in corn-mills, spiral 
or worm conveyors which work on the screw principle being 
very largely used for carr5dng corn. They are not very suitable 
for tanning materials on account of the coarseness of the latter. 



390 PRINCIPLES OF LEATHER MANUFACTURE 

by which the friction is greatly increased ; they are, however, 




Fig. 88. — Chain-convevor. 



occasionally used. Those built up of separate blades are specially 
to be avoided. It must be^ remembered that while the smooth 



GRINDING OF TANNING MATERIALS 391 

small grains of corn behave almost like a liquid, tanning materials, 
and especially rough ones Hke bark and valonia, have a great 
tendency to hang together, and even in hoppers with steeply 
inclined sides will not run down smoothly 'without shaking or 
stirring, but have a great tendency to hang back and form 
arches which do not fall in. Myrobalans is the only material 
which is pretty free from this difficulty. 

An ingenious form of conveyor has been introduced from 
Germany, which consists of a light trough supported on steel 
springs, and vibrated longitudinally by means of an eccentric in 
such a way as to shake the material from one end of the carrier to 
the other, the velocity of motion of the trough being less in the 
outward than the return stroke, so that the material is carried 
with it as it moves forward, and shdes over it in its return. It is 
obvious that the principle may also be applied to screening or 
sifting. 



CHAPTER XXIV 

THE EXTRACTION OF TANNING MATERIALS, AND 
THE MAKING OF EXTRACTS 

Leaching. — The material having been reduced to a suitable 
state of fineness is ready for extraction. This requires a con- 
siderable amount of time, as the tannin is contained in cells of 
which the walls are • of a wood-like substance (cellulose and 
lignine), through which the water diffuses but slowly. Hence, 
unless the material be very finely ground, a long soaking will be 
necessary before it becomes " spent." It should be the aim of 
the tanner to have his barks, etc., ground so finely that they may 
be extracted as rapidly as possible, and yet not be so fine that 
they settle to a compact mass in the leaches and so prevent 
circulation. Using the present methods of extraction on the 
large scale it is necessary to have the material only somewhat 
coarsely ground or crushed, so as to render its percolation prac- 
ticable ; but it is' quite possible that in the future some better 
mechanical means will be found of treating the dust and 
other finely ground matter so as to- bring about a very rapid 
extraction. 

Several patents have been taken for continuous extractors, 
in which the material is fed in at one end of a long rectangular 
tank or trough and gradually moved forward to the other by 
spiral stirrers or some similar device, the extracting liquid being 
continuously run in at the end from which the exhausted material 
is removed, and means being supplied for suitable heating. It 
is possible that some such method may prove practicable for 
the treatment of finely ground material, but at present such 
machines seem either to require too much water and make too 
weak liquors, or not sufficiently to extract the material. Possibly 
they might prove useful in extract manufacture for a preliminary 
or- a final treatment in conjunction with press leaches. An 
apparatus of this sort, under the name of " Automat," is made 
by Messrs Blair, Campbell & M'Lean of Glasgow. If used for 
the fresh material in extract manufacture, the liquors would 
probably require filtration. 

Up to perhaps i8o years ago no attempt was made to leach 
the tanning material, which was simply strewed in layers between 

392 



EXTRACTION OF TANNING MATERIALS 393 

the hides and moistened with water. Leaching originated in 
England, and was first appHed merely to complete the exhaus- 
tion of the material which had been already used for layers ; but 
the use of even weak liquors instead of water in the layers was 
found so advantageous, that new material was soon applied to 
make stronger infusions. The earliest form of leach was simply 
a pit with a perforated wooden " eye " or shaft down one corner, 
in which a pump could be placed to remove the liquor without 
being choked with sohd matter. This was considerably improved 
by the addition of a perforated " false bottom " to the pit, with 
which the eye communicated. The perforations of the latter 
were found unnecessary, and it now serves simply for pumping 
through, or for the manipulation of a plug in a hole communicating 




Fig. 89. — Section of Leach Bottom. 



with an underground " trunk " leading into a pump-well. The 
false bottom is best made of laths about i inch thick and 2 inches 
wide, cut slanting so as to be wider on the upper than the lower 
surface, which makes the spaces between them less liable to 
choke. The laths are nailed on cross-battens with copper nails, 
which should be long enough to clinch, J-inch to |-inch spaces 
being allowed between the laths according to the fineness of the 
ground material. The lattice bottom should be in at least two 
sections, so as to allow of its easy removal for cleaning, and should 
rest on detached blocks, which are best nailed to the underside 
of the battens. A space of 2 inches to 3 inches below the false 
bottom will prove sufficient if it is cleared every time'the pit is 
emptied, but not otherwise. Clearness from obstruction both 
below the bottom and between the laths themselves is very 
important in securing free running in the " press leach " system 
about to be described. A section of the latticed bottom is shown 
in fig. 89. The laths are easily cut by employing a circular saw 
with a tilted table and turning the board at each cut. No 
advantage is gained by planing them. 

As a strong liquor cannot be made by the use of a single 
leaching pit, a series of pits are now always employed, and it is 
the leaching, systematic or otherwise, which determines how 



394 PRINCIPLES OF LEATHER MANUFACTURE 



much of the total tannin will be thrown away and lost in the 
" spent tan." In the case of properly extracted materials the 
" spent tan " will not contain more than i per cent, of tanning 
matter, but the degree of extraction which is profitable is de- 
pendent on the tanning material employed and the class of 



A- 



E 



S 



1^ 



I 




SECTlOrM A-B. 

Fig. 90. — Plan and Section of Battery of Press Leaches. 

leather to be produced. Weak liquors may of course be 
strengthened by evaporation. 

The system of leaches now considered to be the best is based 
on the continuous circulation of the liquor by gravity from the 
weakest to the strongest pit. In its different forms the " press 
leach " is the simplest, and in most cases is aU that is required. 

A plan and vertical section of the leaches is shown in fig. 90. 
Assuming that the leaches have been working for some time, 
and that the liquor in the strongest leach has been run off to the 
tan-pits, or, in the case of manufacturing extracts, to the de- 
colorising tanks or evaporator, the last vat in the series is now 
filled with water or spent liquor, which may be heated by steam 



EXTRACTION OF TANNING MATERIALS 395 

if desired, and this water, which completes the exhaustion of the 
material in this vat, forces the liquor forward in the whole series, 
so that it gets stronger and stronger as it passes from vat to vat. 
The very weak liquor remaining in the last vat is now pumped 
into a spare pit, or on to the next stronger vat, pressing the 
liquor forward as before ; the vat is emptied of the spent material 
and refilled with new, and now becomes the head leach ; and the 
strongest liquor is pressed on to it by running water or weak 
liquor on the weakest vat. 

As regards the construction of such a " battery " of leaches, 
details will differ according to whether the usual English square 
sunk pits or the American form of circular tub leaches is em- 
ployed. In the former case the vertical spouts connected with 
the space under the false bottoms are usually made of wood, like 
the oid-fashioned " eye," and placed at one side or corner of each 
pit, and connected with the top of the next pit by a short trough, 
which may be open above or covered as preferred. Both eyes 
and cross-troughs must be of ample size, so as not to check the 
running of the liquor, and for a set of six or eight leaches the 
bottom of the cross-trough should be at least 10 or 12 inches 
below the actual top of the leach, which should not be filled with 
material above that level. The object of this is to allow of a 
sufficient fall from the first to the last leach. Means must be 
provided for the temporary closing of the cross-trough between 
the vats which form the first and last leach. On a very small 
scale this may be done with a plug ; sliding wooden doors are 
convenient, but difficult to keep tight. A hinged or shding door 
held against an indiarubber facing by a wedge or toggle-joint 
would seem a practicable device. 

If round tub leaches are employed, the vertical connection 
may be similarly made with a wooden trough, but copper tubes 
are almost essential for the cross connections. If a vertical 
copper eye in the centre of the leach be provided for boiling or 
for emptying the leach (p. 398), it may be utihsed for the upflow 
by connecting it with the cross pipe with a thin copper pipe of 
large diameter, which must be movable for the purpose of casting 
the leach. A joint like that of a stove-pipe will probably prove 
sufficiently tight, but if necessary may be made tighter by rolling 
an indiarubber ring over it. 

Six to eight leaches is generally a sufficient number to form 
a press leach " battery." If more are connected in one series 
it wiU usually be necessary to assist the circulation, either by 
pumping an intermediate leach, or by one or more pumps on 
the Holbrook system, in which a power-driven pump of simple 



396 PRINCIPLES OF LEATHER MANUFACTURE 

construction is fitted in the eye of the leach. A simple square 
plunger in a square " eye " with two hinged flaps is aU that is 
needed, and no bottom valve is required, as the lift is only a few 
inches. Such a pump will also serve many purposes of agitation 
and mixing. It is hardly necessary to note that the hquor must 
run downward through the leaches and itp through the vertical 
pipes in order to prevent mixture of the weaker with the stronger 
liquor. This will be rendered the more certain if only the tail 
leaches are heated, as the warm weak liquor will always float on 
the top of the cold. 

Several additions and modifications to the system have been 
made with a view of obviating the so-called " channel difficulty." 
There has been a fear on the part of some tanners that the liquid 
in the leaches may push the material aside and form channels 
through it, thus preventing proper extraction of the tanning 
matter. In the Author's opinion this evil has been greatly 
exaggerated, as, unless the Hquid be pumped from the leaches 
at a very rapid rate while they are in circulation, it is not at all 
easy for the formation of such channels to take place. In any 
case it can be entirely avoided by turning over the material 
in the leaches occasionally, so as to lighten it somewhat and 
rearrange it a little. 

It may also be pointed out that the provision of a proper 
system for pressing or circulating leaches does not prevent their 
being pumped off as frequently as desired, though this is generally 
to be avoided, since when the leach is emptied of liquor the 
material tends to settle into a compact mass, which is not easy 
to percolate, and which is hable to shrink from the sides of the 
pit, thus causing the very trouble which it is desired to avoid. 
In this case, stirring the material with a pole and running the 
leach backwards for a few minutes will sometimes get over the 
difficulty by lifting the material. There are some advantages 
in taking the first and strongest liquors off the material in a 
separate tank, and then finishing the exhaustion in the press 
leaches, since many materials swell, and pack tightly when they 
are first wetted, but on the whole the method hardly pays for 
its added cost, unless it can be done mechanically. On the 
larger scale, the automatic extractor mentioned on p. 392 would 
seem worth consideration, and could be arranged with a chain- 
conveyor to dehver the wet and swollen material to the required 
leach. 

The press leach system as above described is well adapted 
for the requirements of tanners, as its first cost is very small in 
addition to that of the construction of the leaches themselves : it 



EXTRACTION OF TANNING MATERIALS 397 

extracts the bark well, and saves much labour in pumping, and 
greatly lessens the tendency of the pumper to miss pits in the 
series to save time when the master's eye is not on him. 
Another advantage which is often important, is that when the 
leaches are full, much more than a single liquor can be run from 
the head leach without pumping on ; and similarly when they 
are run down to their lowest level, much more than a single 
hquor can be pumped on to the worst leach before it overflows. 
As the leaches flow slowly in comparison to the rate at which 
liquors can be pumped by a good steam pump, it is very advan- 
tageous to allow the pump to discharge into a liquor-tank raised 




Fig. 91 . — Valve for Liquor Troughs. 

to such a height that the liquor can be run from it into any 
leach at a suitable rate for the circulation, and it also enables 
liquors to be pumped without waiting till room has been found 
for them in the leaches. Similar tanks are very useful in running 
liquors for the yard, and especially for the suspenders in a sole 
leather yard, enabling circulation to be kept up during the night 
and at other times when the pumps are not running. They 
may also be used as filters for the suspender liquors by fitting 
them with false bottoms covered with a layer of nearly spent tan. 
The Hquors may be distributed to the different pits and leaches 
by means of canvas hose-pipes, or, what is often more convenient, 
by overhead troughs, carefully levelled, and fitted with discharge 
valves where required. The latter are conveniently made of 
lead in a hemispherical form,i resting on an indiarubber washer 
supported by a light brass casting, or a suitably turned rebate 
in a block of wood {cp. p. 550 and fig. 91). Such valves, if good 
indiarubber is used, wear well, and are absolutely tight. 

In England leaches are usually sunk in the ground, and are 

^ The lead can be cast in a suitably shaped pressed-steel basin, black- 
leaded to prevent its adhesion, and with the wire Unk suspended in it. 



398 PRINCIPLES OF LEATHER MANUFACTURE 

frequently made of brick and cement, or of large Yorkshire flag- 
stones. Such leaches are somewhat costly, but very durable. 
Square wooden pits, puddled outside with clay, are also used, and 
last well with cold, or even warm, liquors, but will not stand 
direct steaming, the wood gradually bending and allowing the 
clay to leak into the liquor, causing black stains. On the con- 
struction of vats with reinforced concrete compare p. 549. The 
large round vats of thick pine, and often holding 10 or 12 tons, 
which are generally used in the United States, stand boiling much 
better, and are frequently supported above a tramway or con- 
veyor, into which the spent bark can be discharged through 
a manhole in the bottom. If this method is adopted, it must be 
remembered that bark, and indeed most other tanning materials, 
will not run through a hole like corn, but must be cast into it, so 
that unless the vat is of great depth it is simpler, and almost as 
easy, to cast over the top. If the manhole is used, either the 
manhole must be almost the size of the pit, or a central hole must 
be made in the false bottom, and this must be surmounted by a 
copper pipe made in sections of 2 or 3 feet, and reaching to the 
top of the leach. When the pit is to be emptied, the top length 
is removed, and the tan shovelled down the hole until the second 
length is reached, and the process repeated. The central pipe 
serves also for the circulation of the liquor when the pits are 
boiled, and may be used as the ascending pipe for circulating on 
the press leach system. It is simply jointed like a stove-pipe. 

In the United States several machines have been introduced 
for the mechanical casting of leaches, one of which is shown in 
fig. 92. The writer has no experience of their efficiency, but 
their use seems to be practicable, and would obviate one of the 
most laborious and unpleasant operations of the tannery. That 
illustrated is made by the Chas. H. Stehling Co. of Milwaukee, 
Wis., but a very similar machine is constructed by the Carley 
Heater Co., Olean, N.Y., and both firms supply brass pitch-holes 
and other necessary parts. The mode of operation will be clear 
from the illustration, the rotating shover being lowered into the 
leach as the pitching proceeds. The same machine can be used for 
evenly spreading the material as the leach is being filled, and of 
course can be shifted from one leach to another as required. The 
pitch-hole is near the side of the leach, and Messrs Stehling inform 
me that it is easily kept open by the insertion of a conical plug 
during the leaching, which is withdrawn for pitching, any slight 
subsequent choking being easily cleared. The leach must of course 
be a round one, but may be constructed of timber or reinforced 
concrete, the latter being recommended by Messrs Stehling. 



EXTRACTION OF TANNING MATERIALS 399 

Leaches even up to 20 feet diameter can be pitched, but in the 
opinion of the present writer 8 feet is a quite large enough 
diameter, and it is better to multiply the number of leaches 
than to make them excessively large. It is also undesirable to 




Fig. 92. — Leach Casting Machine. 



make them extremely deep, as the tendency is to compress the 
material and make circulation difficult. 

The question of the influence of temperature on extraction is 
discussed on p. 412, but except where a pale colour is aU-im- 
portant, it is generally profitable to use a moderate degree of 
heat in extraction. In the opinion of the writer (which is sup- 
ported by a vast amount of careful experiment) only the nearly 
exhausted leaches should be heated, not merely to avoid dis- 
coloration, but to extract the maximum amount of tannin. In 
extract manufacture, where a large quantity of material has to 



400 PRINCIPLES OF LEATHER MANUFACTURE 

be extracted in a short time in a limited number of leaches, and 
where the obtaining of strong liquors is more important than the 
greatest economy of material, much more boiling must often be 
resorted to, and steam coils are better than open steam. In 
American tanneries the boiling is frequently done by copper coils 
fixed below the false bottoms of the vats, but such coils are very 
costly, and, where weak liquors only are to be heated, seem to 
present no advantage over a well-arranged system of heating by 
direct steam, in which care is taken that dry steam only is used, 
and that all water condensed in steam-pipes, and usually contain- 
ing iron, is removed by effective steam-traps. If steam is blown 
into cold liquor through an open pipe a very disagreeable rattling 
and vibration is produced, which is not only annoying, but is very 
injurious to the leaches. This evil may be avoided by the use 
of " silent boiling jets " on the principle of the steam-jet water- 
raiser ; and, following a suggestion of the writer, these jets may 
be used at the same time to circulate the water through the 
tanning material of the nearly exhausted vat, and so wash out 
the last traces of tan. The simplest way to accomplish this is to 
lower the boiling jet, directed upwards, and connected with a 
movable steam-pipe, into the eye of the leach (which is preferably 
central) so that the heated water flows over its top and percolates 
downwards through the material to be washed. Two forms of 
these boiling and mixing jets made by Messrs Korting are shown 
in figs. 93 and 94, and similar jets are now also made by English 
engineers. 

Batteries of closed copper extractors, worked on the press 
system, and similar to those used in extracting sugar from 
beetroot, have frequently been advocated, but are very costly, 
and have no other advantage for tanners over open vats than 
that the liquor can be forced through the series by pressure 
instead of circulating by gravity. No advantage is gained by 
boiling under pressure, since even boiling in open vats has been 
shown to destroy tannin, darken the colour of the liquor, and 
increase the amount of insolubles, and higher temperatures are 
still more injurious. 

Closed vats worked at pressure are, however, much used by 
extract manufacturers, as they certainly enable a higher gravity 
extract to be made from the same weight of material, principally 
by bringing certain woody constituents into solution which are 
not dissolved at lower temperatures. Such extracts frequently 
give the reaction for lignin which is usually characteristic of 
sulphite-cellulose liquors. 

Similar objections would not apply to working such extractors 



EXTRACTION OF TANNING MATERIALS 401 

at so high a vacuum as would cause the materials to boil at a 
low temperature. The boiling would expel liquid from the pores, 




Figs. 93 and 94. — Boiling and Mixing Jets. 



which would again be fiUed with fresh liquor on reducing the 
vacuum, and this would probably lead to very rapid and complete 
extraction, while all necessarv moving of liquors would be just 

26 



402 PRINCIPLES OF LEATHER MANUFACTURE 

as well done by the atmospheric pressure as by steam pressure 
in excess. Where airtight extractors are available, the method 
seems well worth trying. 

Figs. 95 and 96 show a battery of extractors such as are used 
in the manufacture of beetroot sugar. To adapt them for use in 
tannin extraction it would probably be desirable to enlarge the 
discharging manholes, perhaps to the whole diameter of the 
autoclave. 

Heating the weakest leach in the press leach system promotes 
the even circulation of the liquor, since the warm weak liquor is 




Figs. 95 and 96. — Battery of Sugar Extractors. 



much hghter than the colder and stronger liquors in the forward 
leaches, and so floats on the top, and presses the stronger liquor 
uniformly downwards. It also has the advantage that the 
liquors are cooled before they are strong enough for the yard, 
while in tanneries where all the leaches are heated expensive 
tubular coolers are often employed. As the liquor cools, much 
of the colouring matters and reds dissolved in the hot liquor 
separate, and are filtered out by the tanning material, so that 
much brighter and lighter-coloured liquors are obtained. 

Sprinkler Leaches (fig. 97) were formerly used in many tanneries 
and extract factories, especially in the United States. They were 
introduced by Allen and Warren, and yield a liquor which is at 
first very strong, but which becomes very rapidly weaker as the 



EXTRACTION OF TANNING MATERIALS 403 



running is continued. These leaches are similar in principle to 
the mashing-tub and sparger of the brewer, but the process is 
not well adapted for tanners' use, as the material is left too much 
exposed to the air, which is apt to cause oxidation and loss of 
tannin. It is also extremely difficult to completely exhaust the 
material without using an impracticably large volume of water. 
Sprinkler leaches are arranged so as to spray the liquor, or 
water, on to the top of the solid material which is to be extracted 
at such a rate that it flows out just as rapidly as it flows into the 
vat. Some idea of the great amount of oxidation and consequent 
loss of tannin which takes place in this form of extractor may be 
obtained when it is remem- 
bered that this same method 
is now used for the destruc- 
tion of sewage matter by 
spraying it on to beds of 
coke so that it may be mixed 
with as much air as possible 
before it is attacked by the 
bacteria of the coke-beds (see 
p. 568), and also to oxidise 
weak alcohol to acetic acid 
in the "quick vinegar pro- 
cess." 

So far as extraction is con- 
cerned, there is no difference 

in principle between the methods adopted by the tanner and 
the extract manufacturer, though the latter usually works on 
a larger scale, and not unfrequently, in order to increase his 
output or the gravity of his extract, employs a higher tempera- 
ture. This is probably justified by practical considerations in 
the manufacture of extracts from very low-grade materials such 
as oakwood, which only contains 2 to 3 per cent, of tanning 
matter, or even of chestnut wood which is somewhat stronger, 
but it is one of the causes why decoloration of the battery 
liquor is generally necessary. 

Liquors which have been extracted at high temperatures almost 
invariably require decolorisation by the addition of some sub- 
stance which precipitates a small proportion of tannin, but 
carries down with it much of the colouring matter, and of the 
colloidally suspended "difficultly soluble " tannins. The principle 
is quite the same as that of the cook clearing her jelly with 
white-of-egg. 

Dried blood, so-called " blood crystals," is chiefly used as the 




Fig. 97. — Sprinkler-Leach. 



404 PRINCIPLES OF LEATHER MANUFACTURE 

decolorising agent, but a paste of blood-albumen has been placed 
on the market, which is said to be free from several of the dis- 
advantages attending the use of the crude material. 

The liquor to be decolorised is run into a mixing vat fitted 
with a steam coil capable of raising the temperature of the 
liquid to at least 80° C, and usually provided with a simple 
rotary stirring gear. The Uquor, as run into the mixing vat, 
must not have a temperature of more than 48° C. (118° F.) nor 
a strength of more than about 20° Bkr. (sp. gr. 1-020). 

The blood or albumen dissolved in a httle water is added to 
the contents of the vat, which are then well mixed, and the 
temperature is raised to over 70° C, when the albumen coagulates 
and carries down much of the colouring matter. The solution is 
run into another tank, where the precipitate is allowed to settle, 
and the clear hquor is then drawn off for the evaporation. The 
muddy portion, about 8 inches in depth, is pumped through filter- 
presses (which may be cheaply constructed of wood), the clear 
liquors going to the evaporators, and the press-cakes being dried 
for manure. 

In addition to blood-albumen, several other substances, such 
as lead acetate (sugar of lead), salts of alumina, alkaline solutions 
of casein and other albuminous matters, and crushed oil-seeds 
have been employed in the decoloration of extracts, but they are 
by no means so efficient as albumen. 

Decolorising always causes a loss of tanning matter, some of 
this being carried down with the precipitated colouring matter, 
and is for this reason to be dispensed with whenever its use is 
not really necessary. It may often be avoided by careful ex- 
traction at moderate temperatures, and this is especially to be 
aimed at in the case of strong tanning materials, which easily 
5neld battery hquors of much greater strength than 20° Bkr., and 
which thus, if they can be sent direct to the evaporator, save 
cost in evaporation, which is often an important consideration. 

Another method which is frequently used to brighten the 
colour of extracts is treatment with sulphurous acid. Dilute 
sulphurous acid solution may be used for extraction, but a more 
common method is to pass sulphur dioxide gas into the liquor 
before concentration. Sulphurous acid acts partly as a weak 
acid in decomposing compounds of the tannins and colouring 
matters with bases, such as lime, iron, copper, but more actively 
by reducing oxygen compounds and preventing oxidation. 
Bleaching in this way does not actually destroy or remove the 
colouring matters, which are apt to reappear on exposure to the 
air, either in the hquor, or perhaps more often in the leather 



EXTRACTION OF TANNING MATERIALS 405 

tanned with it, so that the gain is frequently more apparent 
than real. If present in any considerable quantities, sulphurous 
acid may also cause inconvenience by its swelling action on 
the pelt, and also by attacking the evaporator, but is mostly 
expelled in concentration. 

Another process should perhaps also be mentioned here, though 
not strictly a means of bleaching. Several tanning materials, 
and notably quebracho and hemlock, contain large quantities of 
" difficultly soluble tannins," which render the liquors made 
from their extracts turbid on cooling. These tannins form 
soluble compounds with alkalies and with alkaline sulphites, in 
the latter case probably setting free the sulphurous acid and 
combining with the base. This has been taken advantage of in 
a patent ^ in which quebracho and other extracts are rendered 
soluble by heating in closed vessels mth bisulphites, sulphites, 
sulphides, or even caustic alkalies, and many " soluble quebracho 
extracts " made on this principle are now on the market. In 
this case, even where bisulphites are used, the greater part of the 
sulphurous acid, after serving its purpose in preventing oxidation, 
escapes in course of manufacture, and the extracts may remain 
neutral or alkaline, but as they are largely used as " bleaching " 
extracts, the presence of SO2 is mostly desired. There is no 
reason that such extracts should not prove serviceable in tanning, 
but it has recently been shown by Paessler that the alkaline 
tannin is not absorbed by neutral hide-powder, and it therefore 
may lead not only to discrepancies in analysis, but, in case of 
drum-tannage, where no acid is naturally present, to failure 
to utilise the whole of the tannin, though, when added to 
ordinary liquors, the acids contained in the latter will set free 
the tannins. Dr Lepetit has shown that quite radical changes 
are produced in the tannin of quebracho by autoclave treatment 
with bisulphites. 

A satisfactory soluble quebracho extract has been made in 
Germany by treatment with sodium carbonate, and subsequent 
slight acidification with a weak acid. 

The use of ferrocyanides has been suggested as a means of 
precipitating iron and copper present in extracts, and it may also 
be pointed out, that with many red-coloured tanning materials, 
such as hemlock and quebracho, the addition of small quantities 
of alum to the tanning liquor effects considerable improvement in 
colour, not only by precipitating a part of the difficultly soluble 
" reds," but by developing the yellow colour of certain colouring 
matters (quercetin, myricetin, etc.) which may be present. Such 
^ Lepetit, DoUfus, and Gansser, Eng. Pat. 8582, 1896. 



4o6 PRINCIPLES OF LEATHER MANUFACTURE 

an addition does no harm in the case of soft leathers, but would 
probably be injurious in a sole leather tannage. 

The liquors, whether direct from the leaches or from the 
decolorising vats, must be concentrated by evaporation to syrupy 




Fig. 98. 



consistency for liquid extracts, or until they will become nearly 
solid on cooling, if a solid extract is required. As has already 
been stated, the action of heat tends to cause a ]oss of tannin 
and a darkening of colour by decomposition and the formation 
of insoluble reds. To reduce this loss to a minimum the weak 
liquors are evaporated with as little access of air and at as low 
a temperature as possible, and these conditions are best obtained 
by the use of steam-heated vacuum evaporators. 

It has been explained in Chapter IX. that the boiling tem- 



EXTRACTION OF TANNING MATERIALS 407 

perature of a liquid is simply that at which the vapour-pressure 
is able to form bubbles in the interior of the liquid, and that it 
therefore may be lowered as much as is desired by reducing the 
external pressure to which the liquids are subjected. In the 
vacuum pan the external pressure of the atmosphere is to a 
large extent removed by means of an air-pump, so that the liquid 




Fig. qq. 



may boil freely at very low temperatures, and, consequently, 
with little injury to the Uquid by heat. Although the boiling 
temperature is reduced in this way, the actual quantity of heat 
required for evaporation is not lessened, and if heat is not supphed 
from outside, water may actually be frozen by the cooling caused 
by evaporation in vacuo, and practically as much steam is re- 
quired to evaporate a quantity of water in the vacuum as in an 
open pan. This quantity, and the consequent consumption of 
fuel, can, however, be much lessened in vacuum apparatus by 
what is known as multiple " effets," or effects. If we imagine 
a Hquid boiling by steam or open fire at atmospheric pressure it 
is producing steam at 100° C, and this steam can be employed to 
boil liquid in a pan where the boihng point is reduced by vacuum 



4o8 PRINCIPLES OF LEATHER MANUFACTURE 

to say 90°, and the steam from this again to boil one with still 
higher vacuum and lower boiling point, the heat of evaporation in 
the first pan being recovered in the succeeding one by again con- 
densing the steam to water, when as much heat is liberated as 
was before consumed. Thus in a single-effect evaporator i lb. 
of coal burnt under the boiler may evaporate 8| lb. of water, 
in a double-effect 16 lb., in a triple 23I lb., and so on, some loss 
being incurred by waste of heat at each effect, till there comes a 
point when the saving of fuel will not repay for the prime cost 
and wear and tear of the apparatus, and it is rarely worth while to 
go beyond three or four effects. Each case must be judged on its 
merits, and in some cases a single effect is commercially the most 
economical. 

As regards the detail of the pan, the oldest form, used in sugar 
concentration, was merely a steam-jacketed pan with a copper 
dome, from which the escaping steam was carried to a condenser 
and then to an air-pump. As the boiling was violent and much 
spray was produced which carried over sugar, it was found 
necessary to introduce a spray-separator between the pan and 
the condenser. In more modern pans the heating is often done 
by a steam-box inside the pan with vertical tubes so as to expose 
a larger surface, and cause a quicker circulation. 

To a considerable extent these forms of pan have been super- 
seded by apparatus in which the liquid is sprayed through tubes 
heated externally by steam, the boiling being almost instantaneous, 
and the liquid being evaporated and carried through the apparatus 
in five or six minutes, during which it is never exposed to an 
ordinary boiling temperature. Perhaps the earliest, and still 
one of the best, of these machines is the Yaryan,^ which is shown 
in fig. 98, and its internal construction in fig. 99. In this 
machine the tubes are horizontal. In the Kestner ^ " Climbing 
Film " evaporator they are vertical, and the liquid is carried up 
in a thin film on the sides of the tubes by the escaping steam, 
and the evaporated liquid is delivered at a considerable height, 
which is sometimes advantageous ; or the evaporation may be 
carried further in a " Falling film " body. These evaporators 
can of course be combined to multiple effects. Several of them 
are in successful use in British tanneries, where they were in the 
first instance installed to concentrate partially exhausted liquors, 
but in some cases are now used for the manufacture of fresh 
extract from mimosa bark and other materials. Single- and 

1 Mirrlees, Watson & Yaryan, Glasgow. 

^ Kestner Evaporator and Engineering Co., Ltd., 37 Parliament Street, 
Westminster, S.W. i. 



EXTRACTION OF TANNING MATERIALS 



409 



triple-effect climbing film evaporators are shown in figs. 100 and 
loi. In some cases the exposure to heat is so short that the 




Fig. 100. — Triple effect 



Fig. ioi. — Single effect. 



evaporator is satisfactorily worked at ordinary boihng temperature 

without vacuum, as no perceptible injury is caused to the hquor. 

Somewhat similar apphances, the "Simplex" and " Multi- 



410 PRINCIPLES OF LEATHER MANUFACTURE 

plex " evaporators, are made by Messrs Blair, Campbell & M'Lean 
Ltd. (Govan, Glasgow), and the latter is shown in fig. 102. 




Fig. 102. 



In the evaporation of very viscous hquids, as, for instance, 
in the manufacture of sohd extracts, these appliances are 



EXTRACTION OF TANNING MATERIALS 411 

found unsuitable for the final evaporation, as the tubes 
are liable to choke, and modified forms of the simple pan 
are adopted, often fitted with stirrers. 

As many tanning materials contain small quantities of acetic 
and other- volatile acids, it is found necessary in multiple effects 
to have not only the tubes but the outer steam casings made of 
copper to avoid corrosion,^ but in single effects this trouble does 
not occur. In single effects worked at atmospheric pressure the' 
steam from the evaporating liquid may often be used for heating 
purposes with great economy. Where vacuum is adopted, the 
exhaust steam from the vacuum engine is frequently used for 
heating the first effect. 

In the case of a solid extract, the evaporation must be carried 
on until it is as thick as can be run from the apparatus. To do 
this satisfactorily, stirrers must be provided to keep the extract 
in motion so long as it is in the pan. The thick, hot liquid 
extract is then run into boxes lined with paper or other suitable 
material, where it is allowed to cool and to solidify. 

The pan for the final evaporation of solid extracts should be 
planned so as to allow of easy cleaning and ready access to its 
interior, so that if accidentally the evaporation is carried so far 
that the liquid will not run out, the clearing of the pan may be 
a comparatively easy matter. It is also important that the 
extract exit should be of large size. Probably a broad and 
somewhat shallow pan, heated merely by a steam jacket, and 
fitted with rotating stirrers, is the most suitable. 

The Use of Extracts in the Tannery. — One of the great attrac- 
tions of extracts is that they save the trouble and cost of leaching, 
and as the extract manufacturer makes this his specialty, he 
can often extract more tanning matter from a material than the 
tanner who has no means of concentrating his weak liquors. 
The extract manufacturer also can employ methods of decolora- 
tion which would be impracticable to the tanner, and so enable 
the latter to obtain better colour than if he employed the raw 
material. By the use of extracts a tanner can strengthen weak 
liquors without trouble, and with definite quantities of materials ;• 
and by using extracts for this purpose he is enabled to 
use up the weaker liquors of his leaches, and so employ more 
water and obtain better extraction of his soHd materials than 
if he used them alone. In the case of very weak materials like 
oakwood, the difficulties of making liquors of sufficient strength 
for tanning without evaporation are so great as to render such 

^ It is very probable that some of the modern acid-resisting forms of 
cast iron might be used. 



412 PRINCIPLES OF LEATHER MANUFACTURE 

materials useless to the tanner for his own extraction, and their 
carriage even for short distances may amount to more than their 
total value. Even with much richer materials extraction 
effects a saving if the carriage is a long one, as it rarely pays to 
import any material containing less than about 25 per cent, of 
tanning matter. Even when the strength of the natural material 
is considerable, as in the case of quebracho, extraction may be 
profitable if from its hardness, or other reasons, the material 
is difficult for the tanner to handle. For long voyages, and 
especially from the tropics, solid extracts are more suitable than 
liquid, as the expense of casks is saved, and the danger of fer- 
mentation is lessened. As it is impossible for the tanner to 
judge by appearance or consistency of the strength or value of 
extracts, they should always be bought and sold on the analysis 
of the particular shipment or parcel by a competent chemist. 
For directions for sampling see Chapter XX. 

Extracts simply require to be dissolved in a suitable quantity 
of water or weak liquor at an appropriate temperature to obtain 
a liquor of any required strength. Some extracts are completely 
soluble in cold water or liquor, but most dissolve better by the 
aid of heat. 40° to 60° C. (100° to 140° F.) is generally sufficient, 
and probably no advantage can arise from temperatures over 80° 
(180° F.). Boiling should be avoided, as it facilitates the forma- 
tion of insoluble " reds," with consequent loss of tanning matter 
and darkening of colour. The extract should be run into the vat 
in a thin stream, and continuously plunged up ; where large 
quantities of extract are to be dissolved, a mechanical agitator 
is advantageous. A " silent boiling jet " (p. 401) may be used, 
fitted into a smali casing immersed in the liquor and open at 
both ends, and the extract run into the current it produces. 

Whether in the manufacture of extracts or for direct use in 
the tannery, the temperature at which tanning materials are 
extracted is of prime importance. It is a common mistake to 
assume that the largest amount of tannin is extracted by boiling. 
Mr A. N. Palmer has pointed out that this is by no means the 
case, but that each material has an optimum temperature of 
extraction, at which more tannin is extracted than at any other, 
though a small additional amount can be obtained by boiling the 
extracted residue. The method adopted in analysis of extracting 
as much as possible at low temperatures, and only raising to 
boiling point when half the required volume of liquor has been 
obtained, is intended to give a maximum extraction of materials 
for which the optimum point is not known, and is a good principle 
to follow in practical leaching. The question has been carefully 



EXTRACTION OF TANNING MATERIALS 413 

investigated by J. G. Parker and the Author/ with results which 
are given in the following tables. For many purposes the colour- 
ing matter which accompanies the tannin is a serious disadvantage, 
and it is usually mostly extracted at the higher temperatures, 
and on this account it is necessary for the tanner who will work 
his leaches economically to ascertain at what temperature he 
can extract the largest amount of tannin combined with no more 
colouring matter than he can permit to enter his leather. Most 
materials are satisfactorily extracted at 50° to 60° C, but as a 
general rule it is best to begin cold or nearly so, and only raise 
the temperature as the extraction proceeds. The tables show 
the percentages of tanning matter, and the amount of colour 
(as measured by Lovibond's tintometer), obtained by extracting 
materials in a Procter's extractor {LJ.L.B., p. 102, L.C.P.B., p. 94) 
so long as any colour or tannin could be obtained. 

1 Journ. Soc. Chem. Ind., 1895, 635. 



[Tables 



414 PRINCIPLES OF LEATHER MANUFACTURE 



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EXTRACTION OF TANNING MATERIALS 415 



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41 6 PRINCIPLES OF LEATHER MANUFACTURE 



o c d 






















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EXTRACTION OF TANNING MATERIALS 417 



t^ 





CO 


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in 


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27 



4i8 PRINCIPLES OF LEATHER MANUFACTURE 



< 

H 



^ h ^ 

OJ d G 

fD O ^ 



O "^ 

^ s 

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CO 



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00 


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6 
o 


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On 



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t^ 


t^ 



o o 



o\ 


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t^ 


CO 


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o 



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rt u 


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fin 









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CO 


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CO 


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rt O Or,-! 



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o ^ I I I 1 I I I I 

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EXTRACTION OF TANNING MATERIALS 



o fl 



419 

a 



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PQ 



420 PRINCIPLES OF LEATHER MANUFACTURE 



o c c 

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EXTRACTION OF TANNING MATERIALS 421 



°§e 




















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422 PRINCIPLES OF LEATHER MANUFACTURE 



o fl g 
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EXTRACTION OF TANNING MATERIALS 42: 



o ili g 

^ ^ s 

o o XI 
^ 'o ,^ 



N 


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CO 


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in 


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w 


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M 


9 





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6 


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6 


ro 

6 


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6 




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c^ 


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fin 






















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Tannin 

Matter 

absorbe 







00 
00 


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6 


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H 


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H 


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6 








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pq 



424 PRINCIPLES OF LEATHER MANUFACTURE 



o fl S 






















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in 


O 


9 


9 


vp 


o 


ip 


O 


9 


o 


CO 


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lO 


H 


<M 


vi) 


01 


■^ 


en 


6 


o o 1^ 


ro 


fO 


lO 


^ 


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o 


VO 


t-> 


CO 


o 






















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Ph 


























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V^ 


H 


t^ 


00 


VO 


9 


M 


ro 


ro 


H 




6 



o 

O-Gq^H 9 9 'op HvO o^MOOp 9 

^y^rjQcbco 6^a^6 6 6 h (si uo 

O^.tjO)'"' HMmHHM 

rHiOJ ^ 

«'o '2^^^9^Tf,ocooicp9 
w ^ Ci:; Q ^1 ^ ^ (^ c^ c, (^j ^ ^^ lo 
pq 

< 

M ^ § S 

H^ cu'c.S.i; o c^M ONco-^-^io^ 6 

Vh oj rt t^ H 



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b£ 



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c 




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■^ 


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Tanning 

Matters 

absorbed 

by Hide. 


o 

03 


H 

6 

ro 


f^ 




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O 






p d 






S.2 






-(-> +-" 








cJ 




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O 














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VO 


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ro 


H 


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N 






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o ^md t^r^t^coo 



3 

o 2 

o o o o oo o-^ 

fp'^lO'^O t^CO O^ >-( rHtel 

1 I I I I I I I 
inooooooo'^ 
H ro-1-iO'O c^oo a^<u 

"o 
PQ 



CHAPTER XXV 

FATS, SOAPS, OILS, AND WAXES 

i 

Fats and oils constitute a large class of substances, of animal or 
vegetable origin, which may be solid, pasty, or more or less viscous 
liquids, but which in the latter case are commonly known as 
" fixed " or fatty oils, to distinguish them from the volatile or 
essential oils, which may be distilled without decomposition, 
and which are the source of most of the odours of plants, and of 
quite different chemical constitution. The term " oil " is also 
applied to various products of mineral origin, and especially to 
those derived from petroleum, on account of their similarity in 
appearance and physical properties to the fixed oils, though, 
chemically, they form a very distinct class. The waxes are 
another group somewhat closely allied to the fats ; and there are 
certain fixed oils, such as sperm oil, which, though very similar 
in appearance and properties to the fatty oils, are chemically 
members of the group of waxes. 

As it is obvious that there is no chemical distinction between 
the fats and fatty oils, except that of melting point, it will be 
convenient to treat them together, especially as what is a solid 
fat in one climate may be an oil in another. Palm and cocoa- 
nut oils are cases in point, as the first is buttery and the second 
a hard fat in this country, though they are both Hquid in tropical 
climates. 

For more detailed information on the chemistry of fats and 
oils the reader must be referred to the Leather Industries 
Laboratory Book, sect, xviii., to chaps, x. and xi. of the Leather 
Chemists' Pocket Book, or to the larger manuals devoted specially 
to the subject by Lewkowitsch, Fahrion, and others ; but a few 
general facts must be recapitulated. 

The true fats contain carbon, hydrogen, and oxygen, but no 
nitrogen. They are all salts of glycerin, with organic acids which 
are generally termed " fatty acids," and which resemble in many 
of their physical characteristics the fats themselves.^ Glycerin 

1 The names of the neutral fats or " glycerides " terminate in " in," 
those of the free acids in " ic " ; thus the fat or oil of oleic acid is olein. It 
is best to confine the termination " ine " to the commercial products, 

425 



426 PRINCIPLES OF LEATHER MANUFACTURE 

is a very weak base, of the nature of an alcohol or sugar, and, 
consequently, when a fat is heated with a solution of one of the 
caustic alkalies, the fatty acid combines with the latter, and the 
glycerin is set free. The salts thus formed are denominated 
" soaps." The reaction with stearin (glycerin stearate), the 
principal constituent of hard animal fats, is shown in the follow- 
ing equation: — 

c- • Sodium Sodium ^, 

btearm i, j x ^ . Glycerm 

hydrate stearate -^ 

(Q,H3,C0 . 0)3C3H5 + sNaOH = 3C17H35CO . ONa + C3H5(OH)3. 
890 120 918 92 

If a soap is treated with an acid stronger than its own, the 
latter is set free, while the new acid combines with the base. 
The following equation, for instance, shows the action of hydro- 
chloric acid on the stearic soap : — 



Sodium 
stearate 


Hydrochloric 
acid 


Stearic acid 


Sodium 
chloride 


C17H35CO . ONa 

306 


+ HCl = 
36-5 


284 


+ NaCl. 
38-5^ 



If any soap be dissolved in hot water, and sufficient hydro- 
chloric or sulphuric acid added to render the solution acid, the 
latter will turn first milky, and (if it be kept warm) the fatty 
acid will finally rise in an oily layer to the surface, which in 
many cases will harden, as it cools, to a solid mass. The amount 
of fatty acid in a soap may be roughly determined by weighing 
25 grm., dissolving in 50 c.c. of boiHng water, and adding excess 
of acid, allowing the reaction to take place in a graduated 
cylinder, or a flask with a graduated neck, in a vessel of boiling 
water. When the fatty acid has risen to the top its volume may 
be noted, and each cubic centimetre may be roughly reckoned 
as 0-9 grm. (For more exact methods cp. L.I.L.B., sect, xvii., 
or L.C.P.B., chap, x.) 

Soaps are insoluble in strong caustic alkaline solutions, and 
therefore saponification (as the decomposition of fats by alkalies 

which are often very different to the pure fats. Thus the " distilled 
oleine " largely used in wool textiles is mainly free oleic acid together with 
hydrocarbons analogous to mineral oils, formed by the breaking down of 
the acid by heat. 

i It will be noted that the combined weight of the glycerin and the 
3 mols. of stearic acid is greater than that of the original stearin by 54, the 
weight of 3 mols. of combined water. 



FATS, SOAPS, OILS, AND WAXES 427 

is called) does not readily take place in them, and for this reason 
the soap-boiler generally dilutes his caustic soda solutions to a 
gravity not exceeding 18° Tw. (sp. gr. 1-090), and separates the 
soap at the end of the operation by the addition of brine, in 
which it is insoluble. An easier method, and one which is often 
useful for the preparation of small quantities of special soaps for 
fat -liquors and the like, is as follows : ^ — 10 lb. of a good caustic 
soda, free from common salt, is dissolved in 4 gallons of water, 
and 75 lb. of oil or fat is warmed to about 25° C. or just suffi- 
ciently to render it liquid, and the soda solution is added in a 
thin stream, with constant stirring, which must be continued 
until the mass becomes too pasty. It is now set aside in a warm 
place for at least twenty-four hours, during which saponification 
gradually takes place. For leather purposes a neutral soap with 
a slight excess of fat is generally advantageous, so that the fat 
may be increased to 80 lb. ; or, in place of this, the operation will 
be facilitated by the addition of 5 lb. of commercial oleic acid. 
If soft soap is desired, 14 lb, of caustic potash may be used in 
place of the 10 lb. of caustic soda. The hardness or softness of 
soaps varies to some extent with the fat used, but potash soaps 
are always much softer than the corresponding soda soaps. It 
is obvious that with soaps made in this way all the glycerin 
remains mixed with the soap. If, on testing, the soap does not 
prove to be free from caustic, it may be re-melted, which will 
generally complete the reaction. Before attempting to work 
with large quantities, a laboratory experiment is desirable, using 
10 grm. of soda in 40 c.c. of water, and 75 to 80 grm. of oil or 
fat. The neutrality or freedom of a soap from caustic alkali 
may be tested by touching a freshly cut surface with an alcoholic 
solution of phenolphthalein, which the least trace of caustic soda 
or potash will render pink. 

If solutions of soaps are mixed with those of salts of the heavy 
metals or of the alkaline earths a mutual decomposition takes 
place, the acid of the salt combining with the alkali of the soap, 
and the fatty acid with the metallic base, to form a metallic 
soap. Most of these soaps are sticky masses, insoluble in water, 
but not unfrequently soluble in turpentine or petroleum spirit 
if previously thoroughly dried, so that some of them have been 
apphed to the production of varnish. Alumina soaps are 
occasionally used to thicken mineral oils, or render them more 
viscous ; and soap solutions in reaction with alumina salts are 
sometimes used to render tannages more resistant to water, and 
for waterproofing textiles. The general reaction of the stearin 
^ Carpenter, Soap, Candles, and Lubricants, p. 144. 



428 PRINCIPLES OF LEATHER MANUFACTURE 

soap with calcium sulphate is shown in the following equation, 
though in practice it is sometimes more complex : — 

Stearin Calcium Sodium Calcium 

soap sulphate sulphate stearate 

2C17H35CO . ONa + CaSO^ = NaaSO^ + (C17H35CO . 0)2Ca 

This is the reaction which causes the curdling of soap by hard 
■ water, and is the basis of the " soap test " {L.I.L.B., p. 30). 

True fats cannot be distilled alone without decomposition. 
When distilled in a current of steam some undecomposed fat 
passes over, but the, greater part is broken up into free fatty 
acid and glycerin ; and hydrocarbons practically identical with 
mineral oils are also formed. 

Fats and oils are insoluble in water, and in most cases only 
sparingly soluble in alcohol, but freely soluble in ether, petroleum 
spirit, benzene, and most other hydrocarbons, as well as in 
chloroform, carbon tetrachloride, and carbon disulphide. Petro- 
leum spirit, often called benzine, is largely used for their extrac- 
tion, and for de-greasing leather, and removing grease from 
clothes. In the laboratory, carbon disulphide or carbon tetra- 
chloride is to be preferred, the latter having the advantage of 
being non-inflammable. Chloroform has lately been adopted as 
a fat -solvent in the analysis of leather by the American Leather 
Chemists' Association. It extracts rather more than petroleum 
spirit. Castor oil is an exception to the rule. Owing to the 
large proportion of oxygen which it contains, it is readily soluble 
in alcohol, and very sparingly in petroleum spirit. Other oils, 
when oxidised, usually become more soluble in alcohol, and less 
so in hydrocarbons. 

Oils vary much in their tendency to " dry," or become con- 
verted into solid or sticky resin-like substances. This tendency 
is greatest in some of the seed oils, and least in olive oil and the 
oily part of animal fats (tallow oil, neatsfoot oil). Sperm oil, a 
" liquid wax," is also very free from this tendency, but all other 
marine oils possess it in a greater or less degree. It is not due to 
evaporation, but to the absorption of oxygen by the " unsatu- 
rated " fatty acids. The tendency to oxygen-absorption, and con- 
sequently to drying (and, in the case of leather-oils, to " spueing "), 
is measured analytically by the " iodine value," the absorption 
of iodine (or bromine) being proportional to that of oxygen, 
while it is much more easily measured. 

There are no simple tests by which the purity of oils can be 
determined, though in a few cases the presence of particular oils 
can be detected. The colour-reactions which used to be relied on 



FATS, SOAPS, OILS, AND WAXES 429 

are mostly due, not to the oils themselves, but to natural im- 
purities which can often be removed by treatment. The mixing 
and adulteration of oils is now a science, and those who practise 
it are well acquainted with the customary tests, and take care 
to adjust their mixtures so as to meet them. Taste and smell, 
with practice, often furnish useful indications, but proper chemical 
examination is the only safe guide, and since it has become 
common, adulteration has a good deal diminished. 

Natural oils and fats are invariably mixtures of the glycerides 
of several fatty acids, and their qualities depend simply on the 
character of these glycerides and the proportions in which they 
are mixed. The fatty acids form several groups, differing in 
their degree of " saturation," ^ or, inversely, in their power of 
taking up oxygen, on which their tendency to drying depends. 
The members of any one of these groups resemble each other 
strongly, differing principally in melting points, density, and 
other physical characteristics. It is possible for a glyceride to 
contain two, or even three, different fatty acids. 

Saturated Fatty Acids. — Stearic acid, C17H35CO . OH, and 
palmitic acid, C^jHgiCO . OH, are the most important. At ordinary 
temperatures they are hard, white, crystalline bodies, and melt 
at 69° and 62° C. respectively. They do not, under ordinary 
circumstances, absorb any .oxygen or iodine, and are very 
little liable to chemical change. Together with oleic acid they 
are the principal acids of tallow and other animal fats, while 
palmitic acid and some lower members of the same group are 
more common in vegetable oils. Free stearic acid is ah im- 
portant constituent of the " distilled stearines " used in curry- 
ing ; while " oleostearine " from pressed tallow consists mainly 
of the neutral fats or glycerides of stearic and palmitic acids. 
The glycerides of saturated acids are hard fats. 

Liquid Fatty Acids, Non-drying.— Oi these, oleic acid is much 
the most common and important, its glyceride, olein, forming 
the liquid part of animal fats, and being the principal constituent 
of vegetable non-drying oils. Olive oil consists almost entirely 
of olein, with a little palmitin. The formula of oleic acid is 
C17H33CO . OH, thus differing from stearic acid in having two atoms 
of hydrogen less. The " bonds " or affinities corresponding 
to these two atoms are linked together, but can separate, and 
attach two atoms of iodine, bromine, or chlorine, or one of 

, ^ A " saturated " compound is one the constituents of which are present 
in such proportions that all the combining affinities of each are satisfied 
by the others. Iodine value, see L.I.L.B.,p. 176, and Journ. Soc. Chem. 
Ind., 1902, p. 454. 



430 PRINCIPLES OF LEATHER MANUFACTURE 

oxygen. 1 The iodine value of pure olein is 83-9 (that is, 100 grm. 
absorb 83-9 grm. iodine), and that of ohve oil about 83. Any 
oil with a higher " iodine value " than olein must contain drying 
oils, though a lower value does not necessarily indicate their 
absence if stearin, palmitin, or other saturated fats are also 
present. 

Other Unsaturated Liquid Fatty Acids. — Of these there are 
several groups, differing in their degree of saturation, and also 
probably in their structure. Their glycerides, together with 
olein, and sometimes palmitin, are the constituents of the seed 
• oils, the drying tendency of which depends on their proportion 
of unsaturated acids, and the particular group to which they 
belong. The fish oils contain a pecuhar group of unsaturated 
acids, together with olein, and usually stearin and palmitin, 
like the other animal fats. Linolenic acid, C^yHggCO . OH, one of 
the acids of linseed oil, has six hydrogen atoms less than stearic 
acid, and therefore three double linkings, and will take up six 
atoms of iodine. Its theoretical iodine value is ,274, while linseed 
oil itself often has an iodine value exceeding 180. The iodine 
value of cod-liver oil is sometimes nearly as high. Both oils 
therefore contain other acids less unsaturated than linolenic. 
Fish oils appear often to contain also fatty acids with four or 
more double linkings, and giving octobromides on saturation 
with bromine. 

The " spueing " of leather is often due to the absorption of 
oxygen and consequent resinification of the oils, and therefore all 
drying oils, however pure, are capable of producing it, though 
some are more liable to do so than others {cp. pp. 469-471). 

Linolenic acid, and probably other allied acids, become con- 
verted by absorption of oxygen into solid varnish-like substances, 
which are important to the tanner as furnishing the principal 
constituents of japans for leather, in which they are now usually 
accompanied by cellulose derivatives (p. 478). The unsaturated 
acids of fish oils seldom give hard varnishes, though menhaden 
oil (p. 447) is sometimes used as paint-oil for outside work. 

Most fats are liable to become rancid by exposure to the air, 
acquiring a disagreeable taste and smeU, and an acid reaction 
from the liberation of the fatty acids. The changes which take 
place are somewhat complex. 

Ricinoleic acid, the fatty acid of castor oil, is of peculiar con- 
stitution, being an oleic acid in which one of the hydrogen-atoms 
is replaced by a " hydro xyl " or OH group. The solubility of 

^ Oxygen is not often attached in this simple way, but as two - OH 
groups or in other more complex groupings. 



FATS, SOAPS, OILS, AND WAXES 431 

castor oil in alcohol has already been alluded to. It does not 
dry, and is an excellent oil for lubricating heavy machinery. 
It is sometimes adulterated with " blown " oils, which are made 
from non-drying or slightly drying seed oils, like cottonseed or 
rape, by blowing air through them in a warmed condition. Under 
this treatment they increase greatly in viscosity and density 
and in their solubility in alcohol, but do not acquire the other 
valuable properties of genuine castor oil. It is a curious point 
that though oils are generally lightened in colour by blowing, 
their fatty acids, if freed, are very dark, and often heavier than 
water. 

The " foots " or sediments which oils deposit on standing 
sometimes consist of animal or vegetable fibres, or mucilage 
combined with water, but often are simply the harder fats, 
stearin, palmitin, etc., which crystallise from the oil on cooling. 
In this case they are re-dissolved on warming the oil. Such oils, 
which like neatsfoot and tallow oils become turbid in cold weather, 
are styled " tender." 

Analysis of Oils and Fats. — Modern methods are almost entirely 
quantitative, depending on the determination of certain " values," 
but as fats are complicated mixtures of many glycerides closely 
allied to each other, it -is not usually possible to separate them 
into their actual constituents, as, in the language of algebra, 
there are generally more " unknowns " than independent equa- 
tions, and we have usually to be content with averages. It may 
reasonably be hoped that with advances in analytical methods 
this difficulty will ultimately be overcome, but in the meantime 
much information about the character of an oil can be deduced 
from the " values." To do this some knowledge of the chemistry 
of fats is essential. 

" Fatty Acids." — This term is sometimes restricted to acids 
of the saturated or acetic series, but for our purpose it will be 
more convenient to apply it to all the acids commonly found in 
fats in combination with glycerin. Most of these acids are 
monobasic (viz. combine with only i eq. of alkali). All con- 
sist of a chain of carbon atoms combined with hydrogen, and 
terminating in the group 0=C— O— H (carboxyl), which gives 
them their acid character. There are a few dibasic acids with 
two carboxyl groups'. 

"Saturated " Acids. — Of these the simplest member is acetic acid, 
CH3CO . OH, and the highest found in ordinary fats is arachidic, 
C19H39CO . OH, found in earth-nut or arachis oil, but between 
them is a complete series, mostly increasing by C^. Usually the 
carbon chain is straight, but sometimes branched, a side chain 



432 PRINCIPLES OF LEATHER MANUFACTURE 

taking the place of one of the side atoms of H. This makes no 
diference in the ultimate composition of the acid, as the sub- 
stituted H is replaced by the terminal H of the side chain, and 
hence such acids are isomeric, and usually differ but little in 
their properties. The most common and important members 
of the series are palmitic acid, C15H31CO . OH, and stearic acid, 
C17H35CO . OH, but some higher members are found in the waxes, 
and butyric acid, C3H7CO . OH, is found in small quantity in 
butter. The general formula of the series is C^Hg^+iCO . OH. 

"Unsaturated" Acids. — While in the saturated acids each 
carbon atom is united to each of its two neighbours by a single 
link^ and its remaining two links are combined with and saturated 
by hydrogen, in the unsaturated acids one or more pairs of carbon 
atoms are united by a double link, and are therefore each com- 
bined with only one atom of hydrogen. This double link is 
not so strong as the single link of the saturated acids, and when 
treated with chlorine, bromine, or iodine, or even with hydrogen 
itself under the conditions of the hydrogenation process (below), 
one of the linkages is broken, and combines with two atoms of 
the new element, in the case of hydrogen being actually con- 
verted into the corresponding saturated acid. Oxygen can also 
combine with the broken hnk, and does so in several ways, thicken- 
ing the oil by producing gummy or resinous products in the 
" drying " oils, or even breaking it entirely in presence of mois- 
ture, with the formation of an aldehyde and a new acid of lower- 
molecular weight. This is not without importance in the theory 
of oil-tannages. The number of double linkages varies in the 
different series of unsaturated fatty acids, rising from one in the 
oleic series to three in the linolenic acid of linseed oil which forms 
hexabromides, and even to four in the acids of some of the 
fish oils. 

Hydrogenation. — The most important advance in the tech- 
nology of fats which has been introduced in recent years is the 
method of " hydrogenation " discovered by Sabatier. Though 
hydrogen will not combine with unsaturated fats or fatty acids 
with the same ease as bromine or iodine, it has been found that 
under considerable pressure, raised temperature, and in presence of 
finely divided nickel as a catalyst, the combination does take place, 
rendering it thus possible to convert any unsaturated fat into a 
saturated one, or raise it to any degree of saturation required. 
Thus whale oil can be converted into a solid fat of the stearin 
type, and at the same time it is so far freed from fishy taste and 
smell that it can be used as an edible fat, as, for instance, in the 
manufacture of margarine. A minute portion of nickel becomes 



FATS, SOAPS, OILS, AND WAXES 433 

dissolved in the fat, but it is so small that it appears to have no 
injurious effect. The importance of the method for soap and 
candle manufacture can hardly be overestimated, and for leather 
manufacture fats as hard as deer tallow or American " oleo- 
margarine " can be produced from any soft animal or vegetable 
fat, or even from fish oil. The process is therefore being very 
largely used. 

The Acid Value is the number of milligrammes of potassium 
hydrate required to neutralise the free fatty acid in a gramme 
of the oil or fat. It is determined by titrating the oil in the cold 
with a N/io alcohohc solution of potassium hydrate, with phenol- 
phthalein as an indicator, until a faint pink is produced. In 
this way the free acids are neutrahsed, but the fats are not 
saponified. 

The acid value varies with the age and rancidity of the fat, 
and is most important in oils used for lubrication, as the free 
acids corrode metals. It is also useful in detecting adulteration 
of tallow with distilled stearine, which consists principally of 
free stearic acid. The acid value of a genuine tallow may vary 
from 3-5 to 50, but for lubrication should not much exceed the 
lower value. 

Saponification Value is the number of milligrammes of potas- 
sium hydrate required to combine with the whole of the fatty 
acids present in i grm. of fat or oil. It is determined by boiling 
the sample with a known excess of alcoholic potassium hydrate 
solution and titrating back the excess. It varies with the 
molecular weight of the acids present, and is therefore a very 
useful characteristic of different oils, and its average value is 
given in a succeeding table (p. 436) for the different oils and fats. 

As petroleum oils have no saponification value and rosin oils 
a very low one, low values frequently indicate adultei^ation with 
these substances. Their actual quantity is determined by com- 
pletely saponifying the fat, and then shaking out the soap solution 
with petroleum ether, which extracts the hydrocarbons, and 
evaporating the ethereal solution. Waxes, and sperm and 
" bottlenose " oils (liquid waxes), naturally contain un- 
saponifiable alcohols, and wool-fat and many fish oils, especially 
that of shark-liver, contain these in smaller quantities. For 
their determination a similar method is used, but ordinary 
ethyl ether must be substituted for petroleum ether, as the 
alcohols are not very soluble in the latter. 

The saponification value of course includes any acid value 
present, and to determine the " ether value," or that of the acids 
combined as glycerides, the acid value must be deducted. 

28 



434 PRINCIPLES OF LEATHER MANUFACTURE 

The Iodine Value is, as has been explained, a measure of the 
unsaturated double bonds of the fat, and is the weight of iodine 
absorbed by loo parts of the fat or oil, and is determined by 
treating a chloroform solution of the substance with a known 
excess of the iodising mixture,and titrating back with thiosulphate 
in presence of potassium iodide and starch. Several different 
iodising solutions are in use which give nearly, but not quite, 
identical values, that of Hanus, which is a solution of iodine 
and bromine in glacial acetic acid, being the simplest, and 
for our purpose the most convenient. Though both bromine 
and iodine are absorbed by the oil, the mixture is deter- 
mined by the analysis in terms of the equivalent quantity of 
iodine, and is so always stated. Saturated fats, such as pure 
stearin or palmitin, have no iodine value, but in natural hard 
fats some olein is always present, of which the iodine value 
is 83-9. This approximates to the iodine value of pure olive 
oil, which is mainly olein, but contains some palmitin, and 
probably also traces of some less saturated fat. In drying oils, 
such as linseed, and in fish oils, the iodine value may rise as 
high as 190 to 200. The higher the iodine value, the more 
liable is an oil to " spue " or produce resinous spots if used for 
currying. This must be distinguished from the tendency to 
cause a white scum or efflorescence on the leather, which is also 
called " spueing," but is generally due to the presence of hard 
fats or free fatty acids. As olein is the only unsaturated fat in 
a genuine taUow or animal fat its quantity may be calculated 
from the iodine value, and the higher its proportion the 
softer is the fat. 

Physical characteristics of fats are sometimes useful in judging 
of purity, and as they are generally rapidly determined, they 
must not be overlooked. 

Specific Gravity (the weight of a cubic centimetre in grammes) 
is the most important of these, and is easily obtained roughly 
by the use of a suitable hydrometer, but much more accurately 
by weighing in a gauged bottle or by a Mohr's balance. As oils 
expand rather rapidly by heat, the observance of an accurate 
temperature is important. 15° C. is that usually adopted for 
oils, but for hard fats a higher temperature, such as the boiling 
point of water, is more convenient. If a bottle gauged for 
water at 15° is used, the comparison wiU be with water at this 
temperature, though strictly an allowance should be made for 
the expansion by heat of the bottle itself. The value of specific 
gravity as a test for oils is much diminished by the fact that it 
can be adjusted by mixtures of low gravity mineral oils and 



FATS, SOAPS, OILS, AND WAXES 435 

high gravity rosin oils. Castor oil, with a gravity of -960, is the 
heaviest of natural oils, and sperm oil, specific gravity -880, the 
lightest. All unsaturated oils increase in gravity by oxidation. 

Refractive Index is more easily and rapidly obtained than even 
specific gravity if an Abbe refractometer is available, but this is 
a somewhat expensive instrument, and is not usually possessed 
by tanners ; and as both specific gravity and refractive index 
vary in almost parallel proportion, little is to be deduced from 
the one which is not equally shown by the other. By combining 
the two, however, and calculating the "refractive constant," 
which is independent of temperature, more characteristic figures 
are obtained, and it has been shown that from this the chemical 
" values " can be approximately calculated, as the refractive con- 
stant depends on chemical constitution, and in general terms is 
raised by unsaturated linkages and lowered by hydro xyls, and is 
thus highest in linseed and unsaturated fish oils and lowest in 
castor and " blown " oils, though both types are of high gravity. 
A pretty full discussion of the refractive constant will be found 
in a paper by the present author in the Journ. Soc. Chem. Ind., 
17, 1890, p. 1021. 

Melting and Solidifying Points. — ^These are often of importance, 
especially in curr5dng greases. The melting point is usually 
shghtly higher than that of resolidification. Many fats become 
thickened and pasty at temperatures much higher than that of 
complete solidification, and in such cases the solidification point 
is indefinite. The melting point of mixed fats is always slightly 
lower than the average of that of the constituents taken separ- 
ately. The melting point of tallows depends in the proportion 
of olein to stearin and palmitin. 

Oils pressed from animal fats usually contain dissolved stearin 
and palmitin, which separates and crystallises out at low tem- 
peratures. Such oils are called " tender," and if used for fat- 
liquoring are very apt to cause a white scum of hard fats on the 
finished leather. Oils for this purpose should be filtered when 
artificially cooled to a low temperature, and should bear being 
cooled to freezing point without becoming turbid. 

The melting point of the free fatty acids is usually much higher 
than that of the fats from which they are derived. 

The following table of constants gives the average values for 
the more common oils and fats, but there is some variation in 
different samples believed to be genuine: — 



436 PRINCIPLES OF LEATHER MANUFACTURE 



Approximate Constants of Oils and Fats 



SP. ' 
Name of Oil. Gr. at 
1^;° C. 


Refrac- 
tive 
Index, 


Saponifi- 
cation 
Value. 


Iodine 
Value. 


Solidifica- 
tion 
Tempera- 




^ 


15°. 






ture. 


Blown Rape . 


967 


1-481 


200 


63 





Castor . 


965 


1-480 


180 


84 


-18 


Raw Linseed 


935 


1-484 


193 


175 


— 16 to —20 


Cod-liver (med.) 


926 


1-485 


185 


146 


,, —10 


„ -(Moller's) 


928 


1-481 




165 




,, (brown) . 


928 


1-482 


185 






Coast Cod (mixed) 


930 


1-482 


185 


172 




Whale . 


931 


1-476 


193 


118 


— 2 


Sardine (Japanese) 


925 


1-479 ' 192 


121 


20 


Seal (pale) 


925 


1-478 


189 


138- 


-2 to -3 


Shark-liver (Scym- 












nus) . 


917 


1-478 


188 


102 


-16 


Mixed Fish . 


929 


1-480 


184 


140 




Cottonseed 


925 


1-475 


193 


108 


— 12 


Arachis (earth-nut) 


922 


1-474 


192 


95 


— I 


Maize 


922 


1-477 


190 


118 


— 10 


Sesame (Gingeli) 


921 


1-475 


190 


108 


- 5 


Olive . 


916 


1-470 


193 


86 


— 2 


Rape (Colza) . 


915 


1-474 


176 


98 


— 2 


Neatsfoot 


915^ 


1-474 


193 


70 


Tender 


Lard 


912 


1-472 


193 


79 


-8 to +6 


Sheep-skin grease oil 


917 


1-468 


197 


60 


Tender 


Egg-yolk oil, hen . 


914 


I -471 


184-190 


68-81 


+ 20 


Sperm and Bottlenose 


880 


1-468 


123-135 


67-81 


Dyson's 
spermaceti 


Mineral leather oils -^ 


>5-92 


1-47-51 


Nil 


up to 20 


Rosin oils . . -c 


6-99 


1-50-5^ 


Resin 


43-48 




a 


t6o° 


at 60° 


acids 






Mutton tallow 


895 


1-442 


195 


40 


44 to 45 


Beef tallow . 


901 


1-442 


196 


42 


36 „ 38 


Bone fat 


894 


I-45I 


191 


51 


15 » 17 


Distilled stearine . 


865 


1-445 


Variable 




45 „ 53 


Paraffin wax . 


776 


1-434 


Nil 


4 


40 „ 55 



Non-drying Fats and Oils 

Tallow (Fr, Suif ; Ger. Talg) is the fat of various mammalia, 
principally of the ox and sheep, but occasionally also of the 
goat. The mixed fat obtained from all parts of the carcass is 
known as " rendered tallow," while that obtained from the region 
of the kidneys (suet) is harder. A substance commonly referred 



FATS, SOAPS, OILS, AND WAXES 437 

to as " pressed tallow " or " oleo-stearine " is obtained by press- 
ing ordinary tallow, in cloths, in the hydraulic press. The more 
liquid portion which is expressed is tallow oil, the finer qualities 
of which are used in making margarine. Oleo-stearine must 
not be confounded with the " distilled stearine," obtained from 
Yorkshire grease by distillation and pressure (p. 439), nor with 
candlemakers' " stearine," which is a mixture of free stearic and 
palmitic acids. 

Pure tallow is white and tasteless, but much of that sold 
is yellowish and of a disagreeable, slightly rancid flavour. 
Mutton tallow is usually harder and whiter than that of beef. 
Goat tallow has a characteristic odour, as have the recovered 
stearines and other waste greases from glue-works. Buck tallow, 
which is particularly hard, has now been largely replaced by 
oleo-stearine. 

Beef tallow melts at about 40° C, mutton tallow at 45°. 

In chemical composition, tallow consists chiefly of a mixture 
of the tri-glycerides of palmitic, stearic, and oleic acids, its hard- 
ness diminishing with the increase of the last. 

Tallow should, when melted, be perfectly clear, turbidity 
indicating the presence of water or other foreign matters, due 
either to carelessness in the manufacture or, possibly, adultera- 
tion. Traces of phosphate of lime, or fragments of animal tissue, 
may be present as accidental impurities ; lime, on the other 
hand, is sometimes added to thicken the tallow and enable it to 
retain more water ; starch, china clay, whiting, heavy spar, etc., 
are also occasionally employed. Tallow has been not infre- 
quently adulterated with the distilled fatty acids from wool 
grease. When this is the case, crystals of cholesterol (see L.I.L.B. , 
p. 181) may be detected by examination of the unsaponifiable 
matter of the mixture under a microscope. Such adulteration 
would also give the tallow an unusually high " acid value." 

Methods for the proximate analysis of tallow are given in the 
Laboratory Book, pp. 189 et seq. nd in the L.C.P.B., ch. xi. 

The fats produced by the boiling of fleshings for glue, and 
by the pressing of sheep-skins, are of the nature of soft tallows. 
If the fleshings are delimed with acid and boiled fresh the 
grease is generally of good colour and with little unpleasant 
odour, but contains traces of free fatty acids derived from the. 
decomposition of the lime-soaps. If the fleshings have been 
dried and the lime carbonated, the grease will generally be 
brown, and more or less rancid ; but the lime-soaps are not 
decomposed, unless the " scutch " or refuse be treated with acid, 
when a further yield of grease is obtained. Carbonic acid will 



438 PRINCIPLES OF LEATHER MANUFACTURE 

neutralise lime, but will not decompose lime-soaps. The grease 
from sheep-skins is generally somewhat brown, and often smells 
of the volatile acids and other constituents of the tan-liquors, 
especially if larch bark has been used. These greases are usually 
much improved in appearance and odour if well washed by 
boiling or steaming on water, or by blowing a mixture of air 
and steam through them, or sometimes even by mere heating to 
a sufficient temperature to evaporate the water and drive off the 
volatile matters. By allowing the grease to cool slowly, so as 
to favour crystallisation, till it is of a soupy consistency, and 
then pumping through a filter press with woollen cloths, the 
more liquid part is separated easily from a more solid portion, 
and both may in many cases be used in leather manufacture, 
the tallow for currjdng, and the oil in place of neatsfoot oil. 

Horse-fat, and especially that from the fatty portions of the 
neck (Ger. Kammfett), as well as various other animal greases, 
are used in the manufacture of leather. They differ from taUow 
chiefly in that they have a lower melting point, and contain 
more olein in proportion to the stearin and palmitin than true 
tallow, and are consequently somewhat softer. Though often 
almost white, these greases are sometimes darkened in colour 
by the products of putrefjdng animal matter, but this does not, 
as a rule, interfere with the oil being used for leather dressing. 
They are usually so cheap that they are but little adulterated ; 
means of determining their purity are, however, given in L.I.L.B., 
p. 191. 

Neatsfoot oil is a yellowish, nearly odourless, oil of bland 
taste, which is largely employed in the dressing of calf-kid. It 
has a similar composition to tallow oil and the other oils obtained 
by subjecting the soft animal fats to great pressure at a low 
temperature. It is often adulterated with bone oil, lard oil, and 
cottonseed oil, and occasionally with mineral oil and recovered 
wool grease. 

As neatsfoot oil is somewhat costly, curriers may with 
advantage often use ordinary animal greases (horse-fat, etc.) 
after they have had the harder tallow extracted by cooling and 
pressure, the product thus obtained being, chemically, the same 
as neatsfoot oil, and in every respect as suitable, while it is 
much less liable to adulteration. 

The true neatsfoot oil is prepared by boiling the feet of cattle, 
and sometimes of sheep and horses, with water, and skimming 
off and clarifying the oil which is thus obtained. For use in 
fat-liquors it should be " racked " or filtered at freezing 
point. 



FATS, SOAPS, OILS, AND WAXES 439 

The physical and chemical characteristics of this oil are de- 
scribed in L.I.L.B., p. 192. 

Wool-fat (Fr. Suint, cesype ; Ger. Wollschweissfett) is a grease 
of high specific gravity, exuded from the sebaceous glands 
of the sheep, together with organic salts of potassium. It is 
obtained by extracting wool with solvents ; or by washing with 
alkaline solutions, from which it is recovered by precipitation 
with acid, and subsequent hot-pressing of the " magma," or, more 
recently, by evaporating the scouring liquor to small bulk, and 
centrifuging. Wool-fat is characterised by its low percentage 
of glycerides, the fatty acids which it contains being mainly com- 
bined with higher alcohols (bodies of alcoholic structure, but of 
a waxlike consistency), and chemically it is rather a wax than a 
true fat. Among the alcohols which it contains is included a 
marked percentage of cholesterol and isocholesterol. It is diffi- 
cultly saponifiable, requiring to be heated to 105° to 110° C. with 
alcoholic potash under pressure ; and even then about 44 per 
cent, of higher alcohols remain, which are incapable of further 
saponification. Care must therefore be taken not to assume 
that unsaponifiable matter in greases which may contain wool-fat 
is necessarily mineral oil. For details of analysis see L.I.L.B., 
p. 194. 

Pure wool-fat is nearly white, of salve-like consistency and 
very slight smell, with a density of 0-973 at 15° C. Crude wool- 
fat is yeUow or brown, with an unpleasant and very persistent 
characteristic smeU. Both the pure and the crude wool-fat have 
an extraordinary power of emulsifying with water, which makes 
them very valuable as substitutes for degras in stuffing greases. 
Lanoline and several other preparations under different names 
are mixtures of purified wool-fat and water, of which lanoline 
contains about 22 per cent. 

" Yorkshire grease " differs from crude wool-fat in being 
recovered from the waters employed in scouring wooUen cloths 
as well as wool, and hence contains the free, fatty acids of soaps 
used in scouring, as well as the " oleines," etc., used in oiling the 
cloth, and although it often contains much wool-fat, it is occasion- 
ally destitute of this substance, 

Holden Fat consists of ordinary wool grease mixed with fish 
oil, and is used either as a substitute for or in admixture with 
degras {q.v.): 

Distilled Wool Grease is produced by distilHng crude Yorkshire 
grease with steam. Most of the glycerides are brolien up, but 
many of the free fatty acids, alcohols, and waxes distil over un- 
changed, though a considerable part is decomposed into volatile 



440 PRINCIPLES OF LEATHER MANUFACTURE 

hydrocarbons strongly resembling mineral oils. The distillate is 
separated by coohng and pressure into a liquid " oleine " and a 
solid " stearine." The latter forms a very valuable stuffing- 
grease, which in England largely takes the place of the " oleo- 
stearine " used in the United States, with which, however, it 
must not be confounded. 

Distilled Stearine, prepared as above described, is a pale yellow 
or brown fat, which varies in hardness and in its melting point 
according to the conditions of its preparation. It has a charac- 
teristic odour which is very persistent, and it consists largely of 
free stearic and palmitic acids, most of the Hquid hydrocarbons 
formed by distillation being removed with the " oleine." 

Olive Oil (Ft. Huile d'olive ; Ger. Olivenoel, Baumoel) finds 
extensive use in leather dressing, and especially in the manufac- 
ture of " fat-Hquors " (pp. 378, 471). It is extracted from the 
fruit of the olive tree by. pressure, and of late years from the 
residues by extraction with volatile fat-solvents. Although 
it chemically resembles tallow and lard oils very strongly, its 
adulteration with these substances may usually be detected, 
at any rate roughly, by the taste and odour of the oil. It is 
principally characterised, from a chemical point of view, by 
containing the glyceride of palmitic but not that of stearic acid, 
and by having a much larger proportion of olein to solid glycerides 
than most of the non-drying animal oils. At low temperatures 
ohve oil soHdifies to a product which can be separated by pressure 
into a soHd taUow-hke fat, and a fluid oil consisting essentially 
of tri -olein. 

Olive oil is the type of a non-drying vegetable oil, but though 
it does not thicken materially on exposure, it becomes rancid 
somewhat rapidly, and is thus rendered unsuitable for lubrica- 
tion, the free acids attacking brass and copper. Unless the 
acidity is excessive it does not appear to injure the oil for leather 
manufacture, and for some purposes is actually an advantage 
as aiding emulsification. Free acids in oils may be removed 
by shaking with sodium carbonate solution. 

Ohve oil always contains some free acid ; which is of impor- 
tance in the preparation of fat-liquors, as it facihtates the pro- 
duction of an emulsion. This quahty may be increased by the 
addition, when necessary, of a little oleic acid or of sulphated oils. 

Ohve oil is frequently adulterated with other vegetable oils. 
Probably the most useful criterion is the iodine value, which is 
raised by the addition of any seed oil except castor. The 
refractometer also affords useful indications. Cottonseed, sesame, 
and arachis (earth-nut) oils are the most frequent adulterants of 



FATS, SOAPS, OILS, AND WAXES 441 

the better qualities, and in many cases may be recognised by 
special tests. 

Castor Oil (Fr. Hiiile de ricin ; Ger. Ricinusoel) is the oil 
expressed from the seeds of Ricinus communis, and is a trans- 
parent, colourless or pale yellowish liquid, having a faint odour 
and a disagreeable taste. At a low temperature it thickens and 
deposits slightly, and at —18° C. it solidifies to a pale yellow mass. 

Castor oil is distinguished from all other natural fixed oils 
by its high density (0-960 to 0-964) and viscosity, and by its 
solubility in alcohol and its insolubility in petroleum ether. 
Genuine castor oil is completely soluble in an equal volume of 
absolute alcohol, or in four times its volume of " rectified spirit " 
at the ordinary temperature. It is practically insoluble in petro- 
leum ether, but can dissolve an equal measure of that liquid. 
• For the purpose of the leather manufacturer, the ordinary 
hot-pressed oil, such as is used for lubricating machinery, is quite 
as good as the more costly cold-pressed oil which is used for 
medicinal purposes. It is generally imported in tins holding 
about 40 lb. of oil. Castor oil, and castor-oil soap made as 
described on p. 427, are very good for fat-liquors, seeming to inter- 
fere with dyeing and glazing less than most other oils. Boots 
oiled with castor oil may be blacked at once, and will take a 
good polish. 

The only oils which are usually mixed with castor oil are 
" blown " or oxidised seed oils, or resin oil. Any other oils would 
so seriously lower the specific gravity as to render their use im- 
practicable. For the detection and estimation of these the 
Laboratory Book should be consulted, or if fuller details are 
required the reader is referred to Lewkowitsch's Oils, Fats, and 
Waxes, or to Allen's Commercial Organic Analysis, vol. ii. 

Sulphonated (perhaps more properly " sulphated ") castor 
oil or Turkey-red oil is now largely used for " fat-liquoring," 
for which it was probably first employed by the author about 
1890. This materia] — which must be carefully distinguished 
from the olive oil preparation which is also used for dyeing cotton 
a Turkey-red colour — is made by treating castor oil with one- 
quarter of its weight of strong sulphuric acid (specific gravity 
1-8), adding the latter in very smaU quantities at a time, and 
taking care that the temperature of the mixture at no time 
exceeds 35° C. The mixture is then allowed to stand for twenty- 
four hours, with occasional stirring, and is washed with its own 
volume of water, allowed to stand until the water has all sepa- 
rated, and the oil is then syphoned off. If desired, the oil may 
be further washed once or twice with a solution of strong brine, 



442 PRINCIPLES OF LEATHER MANUFACTURE 

but this is of doubtful advantage, and should in no case be 
excessive. The washed oil is finally neutralised by the cautious 
addition of about one-hundredth of its volume of strong ammonia 
solution (sp. gr. o-88o). 

If properly prepared, Turkey-red oil (sulphonated castor 
oil) will, when largely diluted with water, bear the addition of 
ammonia to alkaline reaction without showing any turbidity 
even on standing several hours. If a turbidity is produced, it 
indicates that the castor oil used was impure and contained some 
oil rich in stearin. 

The alcohol test described on p. 441 may also be applied, as 
the oily layer wiU be entirely soluble if castor oil alone was used 
in the preparation of the red oil. 

Turkey-red oil usually contains about 50 per cent, of fatty 
acids (Allen). 

Linseed Oil (Fr. Huile de lin ; Ger. Leinoet) is used by leather 
manufacturers in the preparation of the japan for making " patent 
leather " (p. 478), and to some extent also in currying, for oiling 
off levants and moroccos, though for these purposes it has been 
largely superseded by mineral oils. It is obtained from the 
seeds of the flax plant, Linum usitatissimum, chiefly grown in 
Russia and India. The Russian seed is usually mixed with that 
from hemp, which also yields a drjdng oil, to the extent of about 
20 per cent., while that from India, being grown as a mixed 
crop with mustard and rape, is never perfectly pure. The Baltic 
oil is considered best for japans, and is improved by storing for 
a considerable time in tanks in a warm place. 

When obtained by cold pressure of the seeds, previously ground 
and damped, linseed oil is of a bright yellow colour ; if a higher 
temperature be used in the extraction the oil is more or less 
brown, and tastes much more acrid. On exposure to air linseed 
oil turns easily rancid, absorbs oxygen, and if spread out in a 
sufficiently thin film it dries to a neutral substance (linox}^!), 
which is insoluble in ether, but pretty soluble in alcohol. This 
property is the one on which the chief value of linseed and other 
" drying oils " depends. 

Linseed oil is chiefly adulterated with other seed oils, cotton- 
seed being the most often used for this purpose, though men- 
haden and various other fish oils are occasionally employed. 
As the density of raw linseed oil varies between 0-932 and 0-936 
at 15° C, the addition of other seed oils or of mineral oil would 
cause an appreciable lowering of this figure, whilst rosin or rosin 
oil would raise it. A judicious admixture of both mineral and 
rosin oils would give a product of normal density, but would 



FATS, SOAPS, OILS, AND WAXES 443 

much lower its iodine value. Fish oils can be detected by their 
characteristic smell, especially on warming. 

Various methods have been proposed for judging the quality 
of linseed oil, but none of them are perfectly satisfactory. 
The best oil is that which dries the most perfectly ; but the 
rapidity of the drying, and the consistency of the dried product, 
are most important factors which must also be taken into 
account. The iodine value, which is a measure of the drjnng 
power, should not fall much below 180. 

A satisfactory practical test, recommended by Allen, ^ consists 
in mixing the oil with three times its weight of genuine white 
lead, and covering a perfectly clean glass surface with the paint. 
An exactly similar experiment is made simultaneously with a 
standard sample of linseed oil, and the rates of drying and the 
characters of the coating of paint compared. 

J. Muter has simphfied this test by merely flooding a plate of 
glass with the oil and then exposing it to a temperature of 38° C. 
(100° F.) in a good current of air. The time required for drying, 
to such an extent that the coating will not come off when lightly 
touched, is noted, and compared with standard samples of oil. 
By applying the finger at intervals to different parts of the film 
surface the progress of the dr5^ng can be readily observed. ^ 

Boiled Oils. — The capacity of linseed oil for thus drying is 
much enhanced by heating, with addition of " driers," to a tem- 
perature of 130° C. and upwards, while passing a current of air 
through the oil and then increasing the temperature until the 
oil begins to effervesce (" boil "). Large quantities of linseed 
oil are now treated in this way for use in the arts. The driers 
used are metallic salts, principally those of lead and manganese, 
which apparently act as oxygen-carriers. Litharge was formerly 
most commonly used, but its place has been taken to a consider- 
able extent by acetate, borate, and resinate of manganese. From 
I to 2 per cent, of either Utharge or manganese borate may be 
used, though less quantities produce a marked effect. Apparently 
litharge gives the most rapid drying, and manganese a much 
paler colour.^ Linseed oil is usually darkened by boiling, and 
increases both in actual weight and in specific gravity and vis- 
cosity. The chemical reactions which take place in boihng are 
not well understood, but it is in the main a process of oxidation 

^ Commercial Organic Analysis, ii. p. 122. 

^ Kathreiner stated that this method is a useful test for fish and hver 
oils, those which dry jnost rapidly being specially liable to " spue." 

* Cp. F. H. Thorpe, Abst. Jour. Soc. Chem. Ind., 1890, 628, from Tech- 
nology, Quart., iii. pp. 9-16; also p. 479 of this book. 



444 PRINCIPLES OF LEATHER MANUFACTURE 

and polymerisation, perhaps accompanied by the formation of 
anhydrides of the fatty acids, and a portion of the drier remains 
dissolved in the boiled oil. These driers may be detected by 
boiling an ounce or so of the oil with dilute hydrochloric acid, 
allowing the mixture to separate into two layers and then syphon- 
ing off the lower into another vessel, and testing for metals (lead, 
manganese, zinc) or acids (boric, oxalic, etc.). 

Black japan for patent leathers is made by boiling linseed 
oil, without blowing air through it, for at least seven or eight 
hours with Prussian blue or with oxides of iron. The japan is 
brownish rather than blue in colour, and it is probable that the 
Prussian blue serves merely as a source of iron oxide, which acts 
both as a colouring matter and a drier. Other driers, such as 
litharge, are sometimes added, and for coloured enamels other 
pigments are substituted for the Prussian blue. Most japans 
now contain nitro - cellulose dissolved in suitable solvents 
(see p. 483). 

Cottonseed Oil (Fr. Hiiile de coton ; Ger. Cottonoel or Baum- 
wollensamenoel) is now expressed in enormous quantities in the 
United States, on the continent of Europe, and in Great Britain. 
The crude oil contains a very characteristic colouring matter 
which, though naturally ruby-red, is sometimes so intense as to 
make the oil appear to be nearly black. This colouring matter 
causes the oil to produce stains, and is therefore removed by a 
process of refining, and a product of a straw- or golden-yellow 
colour is thus obtained. The refining is usually effected by 
shaking the crude oil with a cold 5 per cent, solution of caustic 
soda, using about ten times as much oil as soda solution. 

Cottonseed oil is, on account of its price, seldom or never 
adulterated, but is itself frequently employed as an adulterant of 
olive and neatsfoot oils. It is a semi-drjnng oil, and unsuitable 
for most purposes in leather manufacture. For a description of 
its characteristic properties, both chemical and physical, the 
reader is referred to Lewkowitsch's Oils, Fats, and Waxes, or 
to Allen's Commerical Organic Analysis, vol. ii. 

Sesame Oil (Fr. Huile de sesame ; Ger. Sesamoel ; Teel oil, 
Gingeli oil) is another seed oil, usually of paler colour than cotton- 
seed oil, but resembling it in having scarcely any odour, and 
possessing a bland and agreeable, though not very characteristic, 
taste. It is often used as an adulterant of olive oil. 

Sesame oil is a non-drjang oil, which does not easily turn 
rancid. When present in other oils, it may be detected by 
agitating ro c.c. of the sample with 5 c.c. of concentrated hydro- 
chloric acid in which o-i grm. of white sugar has previously 



FATS, SOAPS, OILS, AND WAXES 445 

been dissolved. After shaking together for at least ten minutes 
the oil and acid are allowed to separate, when, if sesame oil be 
present, the acid layer will have a marked rose colour, the in- 
tensity of which increases with the amount of sesame oil in the 
sample (Baudouin's test). 

Sesame oil is largely used in India for oiling tanned sheep- 
and goat-skins (" Persians "), and has the characteristic property 
of being assimilable in large quantities by leather without the 
latter appearing oily. East India tanned skins often contain 25 
and even 30 per cent. The oil is applied to them in the wet 
condition before they are dried. It is easily detected in the oils 
extracted from these skins by Baudouin's test. The oil seems 
well adapted for many purposes in leather manufacture. 

Cod Oil (Fr. Htdle de morue ; Ger. Leberthran) is by far the 
most important oil used by leather manufacturers, and is obtained 
from the liver of the common cod-fish {Gadus Morrhua) and 
several other members of the genus Gadus. The chief seats of 
the cod fishery are the coasts and banks of Newfoundland, Nova 
Scotia, the Gulf of St Lawrence, the coasts of Norway, Denmark, 
and Germany, the Dogger Bank in the North Sea, and the 
shores of Alaska in the Pacific Ocean. 

The oil was formerly obtained By keeping the hvers of the 
fish in large wooden vats, stirring constantly until so much 
decomposition had taken place that the cells containing the oil 
burst, and the oil thus released rose to the surface and was 
skimmed off with wooden ladles. The crude oil was allowed to 
deposit any suspended matters by sedimentation in a tank, and 
was then poured into casks ready for sale. The " brown oil " 
so often used by tanners was obtained by boiling the solid matter 
left after extracting the oil as above in iron tanks until all the 
water had evaporated ; the oil thus liberated was then strained 
off, clarified, and put into barrels. 

The purer qualities of cod-liver oil are now obtained by boiling 
the livers with water and skimming off the oil which rises to the 
surface. Three grades are on the market at the present time : 
medicinal, or ordinary bright ; an inferior " light brown " ; 
and " dark-brown," or " tanners' oil." It is probable that 
these steam-extracted oils are much more liable to " spue " 
than those extracted by the old method at a higher temperature, 
since Eitner^ has shown that seal oils extracted at a low tem- 
perature spue badly, but lose the tendency if heated for some 
time to 250° to 300° C. 

Genuine cod oil, as suitable for use in leather manufacture, is 
^ Gerher, 1880, p. 244. 



446 PRINCIPLES OF LEATHER MANUFACTURE 

always more or less brown in colour, of specific gravity about 
0-928, and refractive index 1-482. At present prices it can 
only be adulterated with other fish oils, rosin, or mineral oil, or 
with water, gelatine, or mucilage. Of these, rosin oil and petro- 
leum are the most frequently employed in sophistication. 

An inferior variety of oil, known as " coast cod," made from 
the livers of various fish, such as ling, haddock, and hake, is also 
sold, but, as it is frequently mixed with oils from other fish refuse, 
it has a very poor reputation. 

Cod oil, together with most of the other oils obtained from 
fish livers, has the property of producing an intense reddish-violet 
colour when a drop of strong sulphuric acid is dropped upon 
ten or fifteen drops of the oil contained in a white porcelain 
tray or saucer. The reaction succeeds still better if, instead of 
the oil itself, its solution in chloroform, carbon disulphide or 
tetrachloride is employed. This test, although very useful for 
the detection of liver oils when they are present in oils of a 
totally different character, such as rape or olive oils, does not in 
any way indicate whether a sample of fish oil is pure or otherwise. 
A very similar reaction is given by cholesterol which is present 
in wool-fat. 

Shark-liver Oil (Fr. Huile de requin ; Ger. Haifischthran) is 
obtained from the liver of the " basking shark," or " ice-shark," 
chiefly caught off the coast of Norway ; but the livers of the dog- 
fish, which is a miniature shark, and several allied fish also are 
sometimes substituted. 

Shark oil has been employed in tanneries as a substitute for 
cod-liver oil, but, according to Lewkowitsch and to Allen, it is 
no longer employed in England. From its pale colour it is 
probably principally used to improve the appearance of darker 
oils. According to Eitner,i its use causes leather to "spue" 
badly if not previously heated. 

Shark oil is characterised by the- very notable proportion of 
unsaponifiabl© matter which it contains, which is of the same 
character as that of sperm oil, and not easily removed from its 
soap solution by petroleum ether, and where shark or similar 
oils are suspected, ethyl ether should be used for extraction. It 
gives a strong violet-blue coloration with concentrated sulphuric 
acid, the reaction being even more marked than with cod-liver 
oil itself, and of a bluer violet. 

Whale Oil (Fr. Huile de baleine ; Ger. WaUfischthran) is ex- 
tracted from the blubber of various species of whale, and often 
contains traces of spermaceti, the substance which characterises 
1 Gerber, 1886, p. 266. 



FATS. SOAPS, OILS, AND WAXES 447 

the oil from the sperm whale. This yields on saponification 
higher alcohols, which are found in the unsaponifiable matter ; 
but in ordinary whale oil the total unsaponifiable matter seldom 
exceeds i| to 2 per cent. Whale oil is largely used on 
the Continent for " chamoising " {q.v.), and is consequently 
a constituent of degras. It is much less oxidisable than 
cod, and is now often hydrogenated to a consistent and taste- 
less fat. 

Seal Oil (Fr. Huile de phoque ; Ger. Rohhenthran) is obtained 
from the common rough-coated seal, abundant in the Arctic 
regions. It bears a strong resemblance to both whale and fish 
oils, and cannot be detected in mixtures of these. The Swedish 
" Dreikronenthran " (Three Crown Oil) is a mixture of seal and 
fish oils. As genuine seal oil only contains about | per cent, of 
unsaponifiable matter, its adulteration by mineral or rosin oils 
may be detected by a determination of the matter extracted by 
petroleum ether after saponification of the oil (see L.I.L.B., 
p. 178). It is a curious, and not wholly explained, fact that 
while seal oil extracted from the fresh blubber is a pale yellow, 
that from the blubber adhering to seal-skins is a very dark brown 
or almost black. 

There is no simple test by which the purity or otherwise of a 
sample of oil can be determined, as the dealers know all the best 
tests which the users could try, and fake up their oils accordingly. 
For instance, if petroleum is to be added surreptitiously to a cod 
oil, the decrease in specific gravity of the oil caused by this 
addition would be corrected by the addition of a suitable quantity 
of soap or rosin oil, which would scarcely affect the colour, taste, 
or odour of the sample. The only satisfactory method of 
detecting adulteration is to submit the oil to a complete chemical 
examination, and for this purpose L.I.L.B., pp. 156 et seq., 
L.C.P.B., ch. xi., or the larger text-books already named may 
be suitably consulted. 

Menhaden Oil (Porgie oil. Straits oil) is largely used in certain 
districts as an adulterant or substitute for cod oil. It is obtained 
from the Alosa Brevoordia or menhaden, a member of the herring 
family, about a foot long. The fish is caught on the Atlantic 
coast of America, and is so plentiful that it is very doubtful 
whether cod oil can ever compete with it successfully in price. 
The fish are boiled in steam kettles, the oil squeezed by hydraulic 
presses, clarified, and bleached by exposing to the sun in shallow 
glass-covered tanks. An inferior grade is known as " Bank oil." 
Menhaden oil is chiefly characterised by its very high " specific 
temperature reaction" {L.I.L.B., p. 169), which is about 306. 



448 PRINCIPLES OF LEATHER MANUFACTURE 

Its iodine value is also high. It is not a good leather-oil, being 
very liable to " spue." 

Many other varieties of oil extracted from the bodies, and not 
from the livers only, of fishes are classed as fish oils. Menhaden 
oil is the principal of these ; but Japanese oil, sardine and herring 
oils, and those obtained from the refuse of other fish are scarcely 
less important, though as they are derived from such different 
sources it is not possible to quote any definite characteristics by 
which they may be identified when mixed with more valuable 
oils. They are usually very liable to " spue." 

Fish Tallow, which, according to Eitner, is a good and cheap 
substitute for degras, is the solid grease obtained from different 
kinds of fish oil by subjecting them to a low temperature and 
separating the matter which is thus precipitated, or (as in China 
and Japan) the solid fat which is extracted at the same time as 
the oil from the body of the fish. Formerly fish tallow was only 
obtained from and with Japanese train oil, but it is now obtained 
from whale blubber. This latter yields a very pure form of the 
tallow, which does not need any rectification ; but the Japanese 
variety, which is obtained from fish of the herring family, con- 
tains a sort of fish glue, which greatly deteriorates the quality of 
the product. By careful purification, however, this glutinous 
matter may be removed, and the refined product has none of the 
leather-staining properties so characteristic of the crude tallow. 
The refined tallow is sold in square flat cakes, melts at 42° C, 
and is not quite so stiff as ox tallow. 

Degras and Sod Oil are products of chamois leather dressing 
(p. 459) which are used in currying. Skins are treated with 
marine animal oils and submitted to oxidation, and the surplus 
and partially altered oil is recovered. In the French method 
whale and seal oils as well as liver oils are used, and the oxidation 
is slow and gradual, and the residual oil, being liquid, is recovered 
by pressure, and constitutes moellon, of which the first pressing 
{premiere torse) is the best. This is never sold for currying 
in its original purity ; but, mixed with further quantities of fish 
oils, tallows, and sometimes wool-fat, it constitutes the ordinary 
degras of commerce. The additions, though they lower the 
value, are not to be considered as simple adulterations, since 
the moellon alone would be less suitable for the purpose. After 
removal of as much oil as is possible by dipping in hot water 
and pressing a further quantity is recovered by washing with 
solutions of potash or soda, from which it is separated by addi- 
tion of acid, and constitutes a lower quality of degras. The 
moellon is of such value as a currying material that factories are 



FATS, SOAPS, OILS, AND WAXES 449 

run in which chamoising is carried on solely for its production, 
the skins being oiled and oxidised repeatedly till reduced to 
rags. It is also manufactured by direct oxidation of marine oils. 

In the English method of chamoising liver oils are almost 
exclusively used, and the oxidation is much more rapid and 
intense, the skins being packed in boxes or piled and allowed 
to heat. The product obtained in this way is much more viscous, 
and can only be recovered by scouring with alkalies ; and the 
product, recovered with acid, constitutes sod oil. In many 
English factories a modified method is now adopted, and a 
product recovered by pressure which scarcely differs from moellon. 

An important peculiarity of degras and sod oil is its ready 
emulsification with water, which from its mode of preparation it 
always naturally contains, and which should be present in a 
good degras to the extent of not less than 20 per cent. Such a 
mixture, containing water, is a sort of natural fat-liquor, and is 
absorbed much more perfectly by the skins than an oil alone. 
Sod oils, however, are frequently *' evaporated," or deprived of 
water by heating above 100° C, with the object not only of effect- 
ing a fancied improvement, but of getting rid more completely of 
the sulphuric acid which the water is apt to contain. This makes 
them more homogeneous, and consequently much darker in 
colour. It is not easy to neutralise the acid in an aqueous sod 
oil by direct addition of alkali ; possibly ammonia is best adapted 
for the purpose ; or a suggestion, I think due to Eitner, may be 
adopted, of incorporating a small quantity of a suitable soap. 
In any case, very complete mixture is required. If the sulphuric 
acid used in recovery has been insufficient for complete neutrali- 
sation of the alkali, the sod oil will naturally contain soaps, and 
sometimes also free alkali. Free acid and free alkali are both 
injurious to leather, the former if anything the more so, darkening 
the colour, and even rendering the leather tender. When degras 
is used in mixture with other fats, care should be taken not to 
raise the temperature of the mixture so high as to drive off the 
water, to which a good deal of its special efficacy is due. 

The chemical changes which take place during the chamois- 
ing process are as yet incompletely understood. A large pro- 
portion of the glycerin is dehydrated during the " heating," 
forming acrolein (acrylic aldehyde), to the action of which it is 
very possible that the actual conversion of the skin into leather 
is due,^ while the fatty acids also undergo oxidation. Degras 

^ This is negatived by the discovery of Mr J . T. Wood that good chamois- 
ing can be done by the free fatty acids alone, but the residue is not a 
satisfactory degras (see p. 461). 

29 



450 PRINCIPLES OF LEATHER MANUFACTURE 

therefore always contains considerable quantities of oxidised 
fatty acids, which are sometimes associated with nitrogenous 
products from the skins, and which are soluble in alcohol, but 
insoluble in petroleum ether. To these products Simand gave 
the name of Degrasbildner (degras-former, Fr. degragene), and 
it has been considered a measure of the quality of the degras, 
but its exact value and function is rather doubtful. According 
to Simand, a genuine degras should contain not less than 15 to 
20 per cent, of the degras-former as estimated by his method, 
calculated on the dry oil, and a smaller percentage is also present 
in the original fish oils. (For method of estimation see L.I.L.B., 
p. 182.) It is now known to be simply an oxidised oil product, 
and only accidentally contains nitrogen. 

As the process of degras manufacture is obviously mainly one 
of oxidation, many attempts have been made to produce it by 
direct oxidation of fish oils without the agency of skins, both 
by blowing air through the oil, and by addition of oxidising 
agents such as nitric acid. Eitner states that such oxidised oils 
are more liable to " spue " than the original oils, as they already 
contain large quantities of resinised products ; but this is certainly 
not true of all artificial degras, some of which answers its pur- 
pose perfectly as a currying material, though it is very probably 
justified in other cases. Of course the methods of successful 
manufacturers are kept as profound secrets. 

Degras and sod oil, when deprived of water, are dark and 
viscous oils, of high specific gravity (0-945 to 0-955), and there- 
fore heavier than the oils which have been employed in their 
manufacture. 

Waxes, as has already been stated, differ in their chemical 
character from true fats, in that their fatty acids, which are 
mostly of high molecular weight, are combined, not with glycerine 
but with alcohols, also of high molecular weight and of wax-like 
consistency. Most waxes are solid bodies of high melting point, 
but some oils, especially sperm and bottlenose oils, are chemically 
liquid waxes ; wool-fat contains a considerable proportion of 
waxes ; and many marine oils, such, for instance, as shark-liver 
oil (p. 446), contain waxes in smaller quantity in mixture with 
true fatty oils. 

Sperm Oil (Fr. Huile de cachalot ; Ger. Spermacetioel, Walratoel) 
is obtained from the sperm whale, an inhabitant of the Antarctic 
seas. " Arctic sperm " (Ger. DoegUngthran) is a very similar oil 
obtained from the " Bottlenose whale." These oils are very fluid, 
do not dry, and are excellent lubricating oils for light machinery, 
and also good lamp oils. They contain little if any glycerides. 



FATS, SOAPS, OILS, AND WAXES 451 

and about 40 per cent, of unsaponifiable solid alcohols, which 
are soluble in ethyl alcohol, and must not be confused with 
ordinary unsaponifiable mineral oils, which are frequently used 
as adulterants in mixture with fatty oils to adjust gravity and 
the " saponification value." Mineral oils are liquid, and in- 
soluble in alcohol. Sperm oil is the lightest of ordinary oils, 
its gravity being only about o-88o at 15° C. From its price it 
is particularly liable to sophistication. It is used in leather 
manufacture in the finishing of some fine leathers, and some- 
times as a constituent of fat-liquors. Spermaceti, a wax also 
obtained from the sperm whale, is an occasional constituent of 
leather polishes. 

Bees' -wax (Fr. Cire des abeilles ; Ger. Bienenwachs) is one of 
the most important waxes for the leather dresser. As is well 
known, it is obtained from the honeycomb of the ordinary bee. 
It is a yellowish solid body, fairly plastic when fresh, and of 
" waxy " feel. At low temperatures it is brittle and of fine 
granular texture, and when pure is almost tasteless. It is often 
bleached by repeated melting and exposure to sunlight. As 
wax always contains a considerable amount of pollen, it may be 
identified when in admixture with other substances by means of 
the microscope. 

Bees'-wax is almost insoluble in cold alcohol, but boihng 
alcohol dissolves out the contained cerotic acid, which crystallises 
from it on cooling. The wax is saponified by alcoholic potash, 
but the resulting myricyl alcohol (about 54 per cent.) is not 
capable of further saponification. 

Bees'-wax is frequently adulterated. Water and mineral 
matters (ochre, gypsum, etc.), also flour, starch, tallow, stearic 
acid, japan wax, carnaiiba wax, rosin and paraffin wax, are 
among the substances most commonly used in its sophistication. 

The detection of these, and especially of the other waxes, is 
so difficult that it will not be described here. The reader is, 
however, referred to Lewkowitsch's Oils, Fats, and Waxes for 
further information. 

Carnaiiba Wax (Fr. Cire de carnauba ; Ger. Cearenwachs, 
Carnaubawachs) has come largely into use owing to the advent 
of the coloured leather shoe. As it is a very hard wax, melting 
at 83° C, it has become very popular with boot-polish makers, its 
low price being also in its favour. Carnaiiba wax is an exudation 
from the leaves of Copernica cerifera, a palm indigenous to Brazil, 
and is, on this account, often known as Brazilian wax. It is 
difficult to saponify, and with different experimenters has jdelded 
very varied results on analysis ; it is generally agreed, however. 



452 PRINCIPLES OF LEATHER MANUFACTURE 

that it is a complicated mixture of several of the higher alcohols 
and acids. 

lafan Wax is not a true wax, but a fat consisting mainly of 
the glycerides of palmitic acid, and completely saponifiable. It 
is a pale yellow, hard, waxy substance obtained from the berries 
of a sumach {Rhus succedanea, etc.). At ordinary temperatures 
its specific gravity is exactly that of water, and it melts at 56° C. 
Any admixture with other fats would lower the melting point, 
but japan wax is often adulterated with 15 to 30 per cent, of 
water. It is chiefly valuable to leather dressers as a substitute 
for bees'-wax on account of its lower price. 

Volatile or Essential Oils 

These oils are distinguished from those described in the 
previous section in that they are capable of distillation without 
undergoing any serious amount of decomposition, and are 
chemically of quite different constitution. They occur to some 
considerable extent in Nature, but those of most importance to 
the leather trade are produced by the decomposition of more 
complicated materials. They all have characteristic odours, 
and often considerable antiseptic power. 

Birch Oil is by far the most important of this class of oils so 
far as the leather dresser is concerned, since it is the substance 
which gives to " Russian leather " its characteristic odour. 

The oil is obtained by destructive distillation from the white 
outer bark of the birch Betula alba, and the process by which 
the peasants conduct this is one of the rudest that can be imagined. 
A cauldron is fiUed with dry birch-bark, closed, and heated over 
a fire. The vapours which are evolved are carried, by means of 
a pipe, to another vessel which is buried in the ground, and are 
there condensed. The dark-brown liquid (birch tar) is allowed 
to coo], and the tar which rises to the surface skimmed off. The 
tar is sometimes distilled, and an oil is thus obtained which does 
not give the true birch-oil scent very strongly, though occasionally 
sold as a refined oil. The true odorous substance is evidently 
of very high boiling point, and remains mainly in the tar. 

The birch tar is almost entirely used for giving leathers a 
" Russian " odour, for although it smells somewhat strongly of 
tarry products, the oils causing this smell, are far more volatile 
than the birch scent itself, and therefore disappear on storing 
the leather a short time. Tar obtained from various species 
of pine is sometimes substituted for birch tar, but it may readily 
be distinguished from the latter by the odour and the difference 



FATS, SOAPS, OILS, AND WAXES 453 

in the specific gravity. Birch tar has a specific gravity of 0-925 
to 0-945, whilst fir tar has one of 1-02 to 1-05 ; thus the former 
floats on water, while the latter sinks if it be entirely free from 
enclosed air. Fir tar, too, gives up a yellow colouring matter 
to water shaken up with it, while birch tar leaves the water 
colourless. Birch tar has a distinctly acid reaction, and must 
not be kept in iron vessels (see p. 287). 

The leaves and twigs of American black birch {Betula tenia) when 
distilled with water or steam yield an oil which is practically 
identical with that of GauUheria procumbens (wintergreen), and 
consists almost entirely of methyl salicylate.^ It is clarified, and 
to some extent decolorised, by filtration through woollen blankets 
and redistillation. A ton of brushwood is said to yield about 
four pounds of oil. This oil has quite a different odour to that 
of the real Russian oil, and cannot be used in the scenting of 
" Russia " leather. Sandalwood oil with a little black birch or 
wintergreen oil is sometimes employed for scenting small fancy 
articles, and bears considerable resemblance to the true " Russia " 
leather odour. Black birch, aniseed, sassafras, and various other 
essential oils are occasionally used in small quantities as pre- 
servatives, and to cover disagreeable odour in blood-seasonings, 
cements, and other products used in the leather trade. The 
methods employed for their detection and estimation do not, 
however, come within the scope of a work such as the present 
one. Most essential oils have considerable power as antiseptics, 
and in preventing mildew and the attacks of insects. 

Mineral Oils and Waxes 

This class of bodies is totally different in chemical constitu- 
tion from the true oils and waxes, containing neither glycerides, 
fatty acids, nor alcohols, but consisting of carbon and hydrogen 
only, approximately in the proportion of one atom of the former 
to two of the latter. They occur in underground lakes, from 
which they are obtained by springs or borings ; or in shales, from 
which they are separated by distillation. It is commonly sup- 
posed that they have been formed, at some remote period of 
the earth's history, by the decomposition of animal and vegetable 
matters at a high temperature and under great pressure ^ {cp. 
p. 428). 

1 Methyl salicylate is now made synthetically. It has a pleasant odour, 
somewhat different from that of the natural product. 

^ Oils from wells or springs are technically called " petroleum oils," 
those from shale " paraffin " oils, but, chemically, there is no definite 
distinction. 



454 PRINCIPLES OF LEATHER MANUFACTURE 

The mineral oils and waxes are largely capable of being dis- 
tilled without decomposition, but if heated to high temperatures 
are readily " cracked " or broken up into simpler and generally 
more volatile compounds — a fact which is employed in the pro- 
duction of gas, and the utilisation of some of the heavier products 
for motor spirit. 

They differ greatly in their gravity and boiling point, but 
not much in their ultimate composition, consisting largely of 
saturated or nearly saturated hydrocarbons {cp. p. 429), and hence 
are little liable to oxidation,, and acted on by few chemical re- 
agents. From their constitution they are of course unsaponifi- 
able, and in this way can be separated from fats and oils with 
which they have been mixed. (For particulars of the method 
see L.I.L.B., p. 178, and L.C.P.B., ch. xi.) Attempts have been 
made by oxidation to give them an acid character and fit them for 
soap-making, and by means of emulsifying agents considerable 
quantities can be incorporated in soaps. 

The heavier mineral oils are a good deal used in mixture with 
other oils and fats for stuffing leathers, those of a specific gravity 
of o-88o to o-goo being usually most suitable. They are quite 
incapable of " spueing," and are useful in lessening that tendency 
in other oils with which they are mixed. They have not, however, 
the same affinity for the leather fibre as some of the true oils, and 
are to a certain shght extent volatile, and should generally be 
used in mixture rather than alone. 

Most mineral oils, when held so that a strong hght (dayhght 
or electric hght rich in ultra-violet rays) falls upon them, show 
a green or violet fluorescence or " bloom." This is very persis- 
tent, even when the oil is mixed with a large volume of other 
oils, and is often relied upon as a means of detecting them when 
used as adulterants. The test is, however, not infallible, since 
the effect is due to impurities which may be removed by puri- 
fication, or masked by the addition of such substances as nitro- 
benzene or nitronaphthalene, and it also occurs in the hydrocarbon 
products produced in the distillation by steam of animal oils, 
and is occasionally seen to some extent even in oils which have 
not undergone distillation. 

Vaseline and Vaseline Oil are the most viscous and densest 
of the petroleum oil products. They differ from the sohd 
paraffins in chemical constitution, though their ultimate composi- 
tion is almost the same, and some of them are ring-compounds 
(cyclo-paraffins). They are often useful constituents of stuffing 
greases. 

Paraffin Wax consists of a mixture of hydrocarbons similar 



FATS, SOAPS, OILS, AND WAXES 455 

in chemical constitution to the paraffin and petroleum oils, but 
of higher boiling point, and solid at ordinary temperatures. 
Its hydrocarbons are mostly saturated, and hence very stable 
bodies, and little liable to oxidation. They are completely 
unsaponifiable, and unaffected by boiling with alcoholic potash, 
and in most cases by boihng with strong sulphuric acid, by which 
they may be separated from animal and vegetable waxes or fats 
with which they have been mixed. They are quite incapable of 
resinising by oxidation, or of causing " spueing " in leather. They 
are soluble in petroleum spirit, carbon disulphide, and most of 
the ordinary solvents of fats, but insoluble in alcohol. 

Paraffin wax separates from the liquid oils by crystallisation 
on cooling, and the remaining liquid which adheres is removed 
by hydraulic pressing, as in the case of tallow. The hardness 
and melting point vary according to the extent to which the 
pressing has been carried and the temperature at which it has 
been done. The paraffins of higher melting point are as a rule 
the more costly. 

Pure paraffin wax is a white, more or less hard and brittle 
substance which does not melt so easily as ordinary fats, and is 
on this account used in stuffing certain kinds of leather, harden- 
ing the stuffing grease, and making the leather feel less oily. 
When melted, paraffin wax forms a thin liquid, more resembling 
an ordinary petroleum lamp oil than the viscous vaselines and 
leather oils. On ignition it burns with a bright somewhat smoky 
flame, and leaves no ash behind. It is found on analysis when 
mixed with other waxes or oils in the " unsaponifiable matter " 
(see L.I.L.B., p. 178). 

Ozokerit is a natural paraffin material used for the manufacture 
of ceresin candles, which sometimes occurs in the vicinity of 
petroleum springs, especially in Galicia. It is of pale yellow 
colour when pure, and has then a melting point of about 70° C. 
Its chief impurities are petroleum oils, water, and clay. These 
are removed by melting the ozokerit, decanting off the clear oil, 
and filtering it through fine animal charcoal. If hquid oils are 
present the material is treated with alkali or with strong sul- 
phuric acid, and is pressed before filtering through charcoal. The 
refined product is termed " ceresin," and is of a more waxy and 
less crystalline texture than ordinary paraffin wax. 

The Rosin Oils are derived from resins, and mainly from 
colophony or common pine rosin, by destructive distillation. 
Their specific gravity ranges from 0-96 to 0-99, but their chemical 
composition is very imperfectly understood, and appears to be 
by no means constant. Like the mineral oils they are " unsaponi- 



456 PRINCIPLES OF LEATHER MANUFACTURE 

liable, " but often contain small amounts of soap-forming material 
(rosin acids). 

The detection and estimation of rosin oils is often a matter 
of considerable difficulty, but further particulars on this point will 
be found in L.I.L.B., p, i8o. From their cheapness they are 
considerably employed as adulterants of other oils, and their 
high gravity makes them convenient for adjusting the gravity of 
mineral oils when used for this purpose, as the latter are usually 
lighter than the fatty oils. As currying oils they are not par- 
ticularly suitable, though often employed in stuffing picker bands 
and other heavily greased leathers. They have considerable 
antiseptic powers, and for this reason are useful in leather greases, 
preventing heating and checking mildews. 

Rosin itself is occasionally used as an addition to stuffing 
greases, and is said to increase the waterproofness of the leather, 
and to give it a drier feel. In mixture with about half its weight 
of paraffin wax,*and with a little grease if necessary to soften the 
mixture, it is often used in waterproofing mixtures, which can 
be made to melt at 50° to 60° C. Leather will bear immersion 
in the melted mixture without scalding if thoroughly dried in a 
hot stove at a temperature of not less than 50° C. before dipping. 
Any great increase of the proportion of paraffin wax causes the 
rosin to separate. Rosin consists mainly of free acids which 
easily combine with alkalies and alkaline carbonates in boiling. 
It is hence largely used in the manufacture of soaps on account 
of its cheapness and to render them more soluble in water. The 
rosin acids are not so strong as many of the fatty acids, and rosin 
soaps are therefore somewhat strongly alkaline. Rosin soap, 
precipitated among the ground paper pulp in the rag engine, 
by addition of alum or sulphate of alumina, is largely used as a 
sizing for common papers. 



CHAPTER XXVI 

OIL TANNAGES, AND THE USE OF OILS AND FATS 
IN CURRYING 

The conversion of skin into leather by the agency of oils and 
fats is probably one of the most primitive methods, and is used 
in different ways suited to the skins and fats which are available 
by savage races in all quarters of the globe. In its simplest form 
it consists merely in oiling or greasing the wet skin, and kneading 
and stretching it as it slowly loses moisture and absorbs the fat. 
Under these conditions the fibres become coated with a greasy 
layer, which prevents their adherence after they are once separated 
by the mechanical treatment. At the same time some chemical 
change takes place in the fibre itself, which has a part in its 
conversion into leather varying in importance according to 
the method and fat employed, and of which the chemistry will 
be best discussed after some slight sketch has been given of the 
methods themselves. 

The finest furs are still dressed by fulling or treading with 
oxidisable oils, but the most complete sort of oil-leather is that 
produced by " chamoising " or oil-dressing, a process applied to 
the ordinary " chamois " or " wash-leathers " (now made from the 
flesh-split or " lining " of the sheep-skin), and to the manufacture 
of " buff leather " for military purposes. The process varies 
somewhat according to the character of the leather, but the 
manufacture of the common wash-leather may be taken as a 
type. For this purpose the sheep-splits are freed from the loose 
and fatty middle layer (p. 62) by " frizing " with a sharp knife 
on a beam similar to that used for fleshing (fig. 34, p. 195), but 
much more steeply inclined. The process is rather one of scraping 
than cutting, and was originally adopted to remove the grain from 
the deer-skins which were largely used for glove-leathers, since 
oil-dressing does not easily penetrate a skin with the grain surface 
intact. The fleshes are usually delimed by drenching, but 
removal of fat is unimportant. After being well drained they 
are " stocked " for some time with sawdust till they become 
partially dry and porous, the common " faller " stocks shown in 
fig. 26, p. 164, being generally employed. During the stocking 

457 



458 PRINCIPLES OF LEATHER MANUFACTURE 

care must be taken' that the goods are not overheated by the 
friction produced. When the skins have become opaque from 
the inclusion of air between the fibres they are, according to the 
Continental method, shaken out and oiled on the table, and 
after folding into bundles are put back in the stocks. In 
England the oil is usually added in small quantities, during 
the stocking and becomes rapidly and evenly distributed by 
the motion of the skins. In England cod oil is almost ex- 
clusively employed, but on the Continent a considerable pro- 
portion of seal and whale oils is used. As the goods are apt to 
heat, not only from friction, but from the oxidation of the oils 
employed, they are removed from the stocks at intervals and 
allowed to cool, usually hung on hooks exposed to the air. In 
France this exposure to the air is much more considerable than 
in England, the skins being hung for eight or twelve hours after 
each stocking. The drying rooms are kept moderately warm, 
and a good deal of oxidation of the oil takes place in them, 
which materially affects the character of the product, and especi- 
ally of the residual oil or degras, which is afterwards squeezed 
out of the skins and used for currying (p. 448). Great care is 
required to prevent any parts of the skins becoming dry before 
they are completely saturated by the oil, which would cause 
hard and transparent patches which the oil will not afterwards 
penetrate. After each exposure to the air the skins are oiled on 
the table and returned to the stocks. The stocking has to be 
continued for many hours, even for wash-leather ; and as it 
proceeds the skins lose the smell of limed skin, and acquire a 
peculiar mustard-like odour from the volatile products of 
oxidation of the oils. When the skins are completely saturated 
they are, according to the English method, packed in boxes and 
allowed to heat spontaneously by oxidation of the oils, during 
which great care is required, especially at the outset, that the tem- 
perature does not rise so high as to destroy the skins. To prevent 
this they are removed at intervals from the boxes and spread on 
the floor to cool and then re-packed, and this treatment is con- 
tinued until the oxidation is complete and the skins cease to 
heat. During the heating large quantities of volatile and very 
pungent products are given off, and especially acrolein (acrylic 
aldehyde, from the dehydration of the glycerin), which is exces- 
sively irritating to the eyes. The German method is not unlike 
the English, but in France the packing in boxes is omitted, and 
the oxidation is completed in warm stoves, in which the goods 
are hung on hooks. The heating in this case is much more 
moderate and the oil less thickened, a result which is partly 



OILS AND FATS IN CURRYING 459 

due to the different oils employed, and which leads to differences 
in the subsequent treatment of the leather. 

In the French process the oily skins are dipped in hot water 
and wrung ^ or hydraulic pressed, the expressed oil constituting 
moellon or degras (p. 448), and the skins are afterwards washed 
in a hot soda or potash solution, from which a further portion of 
an inferior degras is recovered. In the old-fashioned English 
method the oil became so thickened that it could not be pressed 
out, and the whole was removed by washing with soda or potash 
solution, from which it was recovered by the use of acid, con- 
stituting " sod oil " (p. 449). Now many English manufacturers 
adopt a modified method, and remove a good deal of their oil 
by pressure. 

Buff leather, much used for military accoutrements, is made 
in a similar manner to chamois, from ox or cow hides, the grain 
of which is frized off with a sharp knife. The bleaching, both of 
buff and chamois, is done by exposing to the sun in a damp 
condition, the skins being watered as required with water or 
fat -liquor, or the alkaline emulsion of degras obtained in washing 
the skins ; or chemically by oxidising agents, such as permanganate 
of potash or acidified sodium peroxide. If permanganate is 
used, the leather is treated in a solution of perhaps 5 grm. per 
litre till of a deep brown colour, and then in a solution of 
sulphurous or oxalic acid till the colour is removed. 

Messrs J. & E. Pullman, of Godalming, made a species of 
buff leather, which they styled " Kaspine," by treating limed 
and drenched hides or skins in a drum with a very dilute solution 
of formaldehyde (" formalin ") rendered alkaline with sodium 
carbonate (Eng. Pat. 2872, 1898. Cp. p. 576). The change to 
leather took place very rapidly, and the leather was afterwards 
treated with soap solutions or fat-liquors to feed and soften it. 
It was almost indistinguishable from genuine buff leather, except 
that it was white throughout and needed no bleaching. It 
found considerable application for military purposes. Similar 
processes are now rather largely used in the production of " wash- 
able " glove-leathers, but the details are jealously guarded trade 
secrets. 

A type of leathers which bear a close chemical relation to 
oil-leathers is that including " Crown," " Helvetia," and fat- 
tanned leathers. The first leather of the sort was invented by a 

^ Wringing is done by forming the skins, in pairs of which the ends are 
turned inside the hnk, into a sort of chain, one end of which is attached to 
a fixed hook and the other to a winch, so that the chain can be tightly 
twisted. 



46o PRINCIPLES OF LEATHER MANUFACTURE 

German cabinetmaker named Klemm, by whom the secret was 
sold to Preller, who manufactured it in Southwark under the 
name of " Crown " leather. Klemm used flour, ox-brains, 
butter, milk, and soft fat, which was made into a paste with 
water, and spread on the limed, drenched, and partially .dried 
skins, which were rolled into bundles, and drummed in slightly 
warmed drums for some hours ; taken out, again dried slightly, 
and coated with the mixture, and again drummed. For thick 
hides the process was repeated a third time, drumming in each 
operation for about eight hours. The leather was used for laces, 
picker bands, light belts, and other purposes where great tough- 
ness and flexibility were required. It was found by further 
experience (if indeed it was not known to Klemm himself) that the 
only really essential ingredients of the mixture were the soft 
fats and flour ; and even the latter could, for some sorts of leather, 
be dispensed with. It was further ascertained that only the gluten 
or albuminous part of the flour was absorbed by the leather, the 
starch serving mainly to facilitate the emulsification of the fats. 
The proportions used in the paste are about seven parts of flour, 
seven parts of soft fat such as horse grease, two parts of tallow, 
four parts of water, and a little salt or nitre to act as an anti- 
septic. Other gfeases, such as mixtures of tallow and oil, can 
be substituted for the horse grease, and pipe-clay or ochre may 
to some extent take the place of the flour, while soap may also 
be added, and fish oils are occasionally used. The similarity of 
the mixtures used to the tawing paste in calf- and glove-kid 
dressing (pp. 247, 252) is obvious, and Klemm had an earlier 
process in which the operation just described was preceded by a 
slight alum tannage, and which was almost identical in its detail 
with the methods now in use for the production of so-called " raw 
hide." On the other hand it is nearly allied to the production of 
" Riems," or raw-hide straps in South Africa, for which a long 
thong is cut spirally from a hide and wound into a sort of skein, 
which is suspended from a crossbar with a heavy weight at its 
lower end, and oiled and twisted, with frequent changes of posi- 
tion, until the water is dried out and the thong is saturated with 
fat, forming a very tough and durable leather. A similar material 
can be made by fulling or otherwise working grease into a raw 
hide prepared for tanning. Eitner examined samples of " Crown " 
leather chemically, by removing the gluten of the flour with 
an alkaline solution, and found that an imperfectly chamoised 
leather remained, which, when restufled v\dth fat, was much less 
full, and carried a much smaller quantity of grease than before. 
Various theories have been proposed to explain the reaction 



OILS AND FATS IN CURRYING 461 

which takes place in the production of oil-leathers. Fahrion 
has shown that chamoising can be done with vegetable drying 
oils such as linseed, and the white Japanese leather used for 
brace tabs is produced with rape oil. Knapp supposed that it 
was merely a case in which the smallest fibrils of the hide were 
coated with the products of the oxidation of oils, and so prevented 
from adhering together, and protected from the action of water 
by the sort of waterproof coating which was formed. This 
explanation is scarcely feasible in the face of the fact that chamois 
leather can be treated even with hot dilute solutions of the caustic 
alkalies without destruction, while cotton fibres waterproofed 
by treatment with drying oils have their coating entirely removed 
by treatment with alkalies. Lietzmann supposed that the whole 
of the gelatinous fibres were removed in the liming and subsequent 
treatment, and that the finished leather consisted only of the 
skeleton of yellow or elastic fibre which exists in the skin, and 
which is remarkable for its resistance to heat, acids, and alkalies. 
Unfortunately for the theory, the proportion of these fibres in 
the entire skin is a very small one, and they exist mainly, if not 
entirely, in the " grain," which is removed by splitting, their 
occurrence at all in the flesh-split being doubtful. We know 
that aldehydes are capable of converting gelatinous substances 
into a material very similar in its power of resisting hot water 
and alkaline solutions to the fibre of chamois leather, but the view 
that acrylic aldehyde, which is derived from the glycerin in the 
heating of the skins, is the active agent is no longer tenable, since 
J. T. Wood has shown that excellent chamois leather can be pro- 
duced -by the use of the free fatty acids of fish oils from which the 
glycerin has been entirely removed. The aldehyde theory is not 
entirely disproved by this, since aldehydes are produced (see 
pp. 432, 449) from the splitting of oxidisable oils at the double 
linkage, and in all cases where perfect chamoising is produced 
intense oxidation takes place, but it must be admitted that it 
seems less probable than formerly, and we have still to look for a 
complete explanation. There is no doubt that the coating of the 
fibres with oil products does take place, and it is possible that 
they enter into some sort of combination with the hide-substance 
which is no longer soluble in alkaUes. Such a coating is probably 
a powerful factor in the leathering of " Crown " leather and other 
similar products which are not washed out with alkaline solutions. 
Knapp proved by treating raw pelt, which had been dehydrated 
with alcohol, with a very dilute alcoholic solution of stearic acid 
that a thin coating of stearic acid on the fibres would confer 
great softness and considerable resistance to water. Even where 



462 PRINCIPLES OF LEATHER MANUFACTURE 

no stearic or other fatty acid is purposely added to alcohol used 
for dehydrating pelt, traces are present from the decomposition 




Fig. 103. — Scouring large Seal-skins by Hand. 




Fig. 104. — Scouring Machine. 



of the natural fat of the skin, and there is little doubt that this 
is the cause why such alcohol leathers are much more difficult 
to wet back again to the state of pelt than would a priori be 
expected, and why hide-powder dehydrated in this way is un- 



OILS AND FATS IN CURRYING 



463 



suitable for use in tannin estimation from its non-absorption of 
water. 

It is not within the scope of the present volume to describe 
in detail the processes used in currying, many of which are purely 
mechanical and of no theoretical interest, whatever their practical 
importance. The leather is usually scoured with stone, brush, 
and sleeker to stretch it and free it from " bloom " and loose tan 




Fig. 105. — Hand Shaving. 



(fig. 103) ; or by machines such as fig. 104 ; and is often reduced in 
thickness by shaving by hand (fig. 105) or by machine (fig. 106). 
In place of shaving, hides and skins are frequently split into two 
or more thicknesses. This is done by various machines, of which 
the " band-knife " shown in fig. 107 is the most important, the 
cutting tool being a thin steel belt stretched like a band-saw, and 
sharpened on one edge by an emery-wheel. The use of machines 
is rapidly superseding hand labour, not only from its lesser cost, 
but often from its more satisfactory quality. 

Something must, however, be said about the function of the 
oils and fats used in currying and their general method of applica- 
tion. It is obvious that the possibility of coating the finest 
fibrils of leather with a fatty layer is not restricted to raw hide. 



464 PRINCIPLES OF LEATHER MANUFACTURE 

but is present, sometimes even in a higher degree, in tanned or 
tawed leathers, in which the fibres are already so far isolated 
as to make the access of the fat easy. Even the possibihty 
of oil-tannage is not excluded where the fibre is not already com- 




FiG. 106. — Shaving Machine. 




Fig. 107. — Band-knife Splitting Machine. 



pletely saturated with other tanning agents, or where these agents, 
from their nature, have not so firm a hold on the fibre as to be 
incapable of being displaced by the action of oil-products. It is 
therefore obvious that we may apply some of the ideas which we 
have formed with regard to oil-tannages to the action of fats 
upon tanned leather. In the first place, it must be remembered 
that gelatinous matters are as a rule insoluble in fats ; and vice 



OILS AND FATS IN CURRYING 465 

versa, that fats are incapable of penetrating dry and solid gela- 
tinous fibres. If the skin becomes dry in the chamoising process 
that part remains raw. It may therefore be concluded that fats 
and oils have little power in themselves of isolating the fibrils, 
and that this must be accomplished by other agencies, since if 
they are still adhering together the fats cannot penetrate them. 
Hence the necessity of moisture, which keeps the fibres soft and 
divisible ; and with raw hide, the importance of powerful 
mechanical treatment, which will work the minute globules of 
fat between the fibrils. In the case of tanned leathers the last 
condition is less important, since the fibres are already isolated 
by the taniiage, and capillarity assists the penetration. Even in 
this case the distribution of the fat is much assisted if it is already 
in a state of fine division (emulsification), and if the surface 
tension (p. 88) between it and water is low, as is the case with 
degras and other partially oxidised oils. On this rather than 
on any special chemical affinity probably depends the importance 
of the " degras-former " and other products of oxidation which 
are present in degras, and the difference in penetrating power 
of different oils. So long as oil remains in an undivided con- 
dition so long can it be squeezed out, and the leather will feel 
and appear greasy ; while, when it is thoroughly emulsified 
and adherent to the fibre, it can no longer be expelled by mechani- 
cal means. No doubt the different power of different tannages 
to " carry grease " without appearing greasy is also related to 
the degree of isolation of the fibrils and their surface tension 
with regard to fats. We may judge that the more readily an oil 
can be emulsified, the more freely and completely it is likely to 
fix itself on the leather fibre. For this reason, and for their 
power of emulsifying other oils, the sulphonated fish oils now so 
largely used on sole leather would probably be useful constituents 
of stuffing greases. 

It is a practically invariable rule that the leather fibre must 
be wet when it is stuffed. The surface tension (see p. 88) 
between the water and the fats is less than that of either with 
regard to air (p. 90) ; and therefore, as the water dries out of the 
small interstices of the leather, the fat follows it in, and gradually 
takes its place. Generally speaking, the amount of water should 
be such that some exudes in minute drops when the leather is 
pinched, that is, that not only the minutest spaces between the 
fibrils are filled, but to a considerable extent even the larger ones 
between the fibre bundles. " 

In " hand-stuffing " the leather is now coated on the flesh side, 
or occasionally on both sides, with " dubbing," which is a pasty 

30. 



466 PRINCIPLES OF LEATHER MANUFACTURE 

mixture of fats usually mainly composed of cod oil and tallow, 
which is applied rather thickly with a brush and smoothed down 
with the fleshy part of the forearm. When such constituents 
are melted together the harder fats dissolve in the oils, and 
as the mixture cools much of the hard fats again crystallises out. 
To make a good dubbing the cooling fats must be stirred con- 
tinuously till cool, as otherwise the mixture separates into little 
globular masses of crystals with liquid oil between them, instead 
of forming a uniform body of salve-like consistency.. The pro- 
portions of the hard and soft constituents of the dubbing should 
be adjusted to the season and to the temperature at which the 
drying of the stuffed leather is to take place, so that, on the one 
hand, the dubbing will not melt and run off, and, on the other, 
that it should not solidify more than is necessary, as only the 
liquid solution which remains entangled among the crystals can 
be absorbed by the leather. The solid crystalline fats remain 
on the surface, and are scraped off by the sleeker in finishing as 
" table-grease," which is generally re-melted and used over again. 
It does not answer in hand-stufQng to carry this re-use too far, 
as the table-grease contains only the harder parts of the fat, 
with a continually increasing proportion of stearic acid, so that if 
a dubbing be made continuously of table-grease and oil, in the 
end little but the latter will be absorbed by the leather, while 
where fresh tallow is used, a portion of its softer constituents 
remains dissolved in the oil. The principal function of the 
harder fats is the mechanical one of retaining the oil on the 
surface of the leather ; and to a certain extent they may 
be replaced by other solids, such as steatite (" French 
chalk "), or perhaps other pulpy materials. The use of a 
portion of soft fat, such as bone-fat, or the better sorts of 
glue-grease, is quite practicable, especially if mixed with the 
harder table-grease. 

The drying of hand-stuffed leather should be slow, to allow 
time for the absorption of the grease ; and the temperature 
should be so regulated as to keep the dubbing in a soft but not 
liquid condition. In winter, if the temperature of the outer air 
te raised sufficiently for this, the drying will be too keen {cp. 
p. 518), and the water will be dried out before the grease is 
properly absorbed. It is therefore best, in cold weather, to 
maintain the ventilation mainly by circulating the air in the 
room, with little admission from the outside, and in extreme 
cases even artificial damping of the air may be advantageous. 
Sometimes the tendency to mildew during slow and warm 
drying is very troublesome. This may be prevented by the 



OILS AND FATS IN CURRYING 467 

addition of antiseptics to the stuffing grease. Carbolic acid 
and creosote are effective, but sometimes objectionable from 
their smell ; rosin oil has considerable antiseptic power, and 
mineral oils also in a less degree. Probably a-naphthol would 
prove an efficient remedy, as it has little odour, and its anti- 
septic properties are very strong, but it has not been tried 
by the writer {pp. Chapter V.). 

In drum-stuffing the conditions differ materially from those 
of hand-stuffing. The goods, in a damp condition, are placed in 
a drum (fig. 108), which has been heated by steam to as high 
a temperature as the leather will safely stand. Cold, damp 




Fig. 108. — Turner's Hot-Air Drum. 

leather may be stuffed in a drum heated to 60° C, and the grease 
may be run in at the same temperature. The grease should 
generally be melted and mixed at a somewhat higher tempera- 
ture. Sometimes steam is merely blown into the drum before 
introducing the leather to heat it to the required temperature, 
sometimes a steam coil is placed in the drum itself. A more 
modern method, which is now largely used in the United States, 
is to heat by hot air, which is circulated by a fan over an external 
steam heater and through the drum. The drum is set in rotation, 
and the stuffing grease in a melted condition is run in through 
the hollow axle, or, if this is not provided, it is introduced through 
the door, and the rotation is maintained for twenty to thirty 
minutes. During the last few minutes the door is frequently 
replaced by an open grating, or cold air is drawn through the 
drum by means of the fan, in order to cool the goods, which are 



468 PRINCIPLES OF LEATHER MANUFACTURE 

set out with the sleeker on the table while yet warm, and dried 
under much the same conditions as have been described with 
regard to hand-stuffed goods. 

In drum-stuffing the hardness of the grease is limited by its 
melting point, which must not be so high as to damage the 
leather, but it may be as soft as is desired. As the grease is forced 
by mechanical means into the interior of the leather there is no 
danger of its running off, but the drying must take place at such 
a terhperature as to keep it at least in a partially soft condition, 
as the drumming only forces it into the coarser spaces of the 
leather, and does not complete its distribution on the fibre. By 
the use of exceedingly hard greases, such as " stearine " (p. 440) 
and oleo-stearine (p. 437), sometimes with additions of paraffin 
wax, it is possible to introduce immense quantities of grease, and 
yet to obtain a leather which will board up to a good colour. In 
America it is not unusual to reckon 100 or even 115 lb. of greases 
to 100 lb. of leather weighed dry after scouring, or estimated 
from its wet weight ; and the whole of this is absorbed, scarcely 
anything coming off in " setting." The leather as it comes from 
the drum is dark brown, but when bent sharply in " boarding " 
to form the grain, after cooling and drying, the very hard and 
crystalline fats crumble into white powder, and the leather 
takes a light and pretty colour. Such leather would of course 
darken at once if it were held to the fire, but would again brighten 
on cooling and breaking up with the " board." Some portion of 
liquid fats, such as degras or fish oil, should be contained in 
the stuffing grease, as the solid fats alone will not penetrate to 
the heart of the fibres, but will leave the leather dry and 
harsh. 

By drum-stuffing it is possible to incorporate solid 'matter 
with the leather, and barytes (ground heavy-spar or barium 
sulphate) was formerly much used for this purpose, but has now 
been abandoned as too easily detected. Organic fillers, such as 
flour, casein, rosin or rosin soaps, might probably be used in some 
cases with advantage ; and most colloidal precipitates would be 
absorbed. Glue is often used as a stiffener in rolling splits for 
insoles. Glucose is still used as an adulterant of leather, but is 
not introduced in the drum, but by painting the goods with 
syrup before stuffing. It not only adds weight, and gives the 
leather a lighter colour than an equivalent quantity of grease, 
but at the same time lessens its toughness and absorbs moisture, 
and ought to be prohibited in England, as it already is in Germany. 
On the detection of adulteration of leather see L.I.L.B., p. 212. 
Drum-stuffing is in this country mainly applied to shoe leathers. 



OILS AND FATS IN CURRYING 469 

but in America, wth the hot-air drum, is coming into increasing 
use for harness, and even belting. 

A method of stuffing is used in Germany for heavy belting 
and the like which appears at first glance to contradict the 
axiom that leather must be stuffed wet. It is called Einhrennen 
(to " burn in "), and consists in first drying at a high temperature 
(50° C.) to ensure the absence of all moisture, and then either 
pouring hot melted tallow over the leather on a table and holding 
it over a brazier to allow the grease to sink in, or dipping it 
completely in a bath of melted tallow. The exception is only 
apparent, because, though the leather is at this stage completely 
saturated with tallow, it is only after wetting and drumming 
that it attains the flexibility due to true stuffing. Similar methods 
are applicable to alumed leathers, and even to chrome leather ; 
and so-called " waterproof "or " anhydrous " leather is made by 
immersing thoroughly dried leather in a bath of 2 parts of rosin 
and I of paraffin, or some similar mixture. If the leather is not 
first thoroughly dried, it is scalded and destroyed by the hot grease. 

The most troublesome defect to which stuffed leathers are 
liable is known as " spueing," and is of two kinds, of which the 
first and less serious (perhaps more properly distinguished as 
" striking out ") consists of a white efflorescence rather like 
incipient mould, which is easily wiped off, but generally reappears. 
This is due to the crystallisation of the harder fats, and especially 
of the free fatty acids, on the surface of the leather, and is almost 
sure to occur in greater or less degree when the hard fats such as 
tallow or stearine are combined with a non-dr3dng oil such as 
neatsfoot, or when soft fats are present in the leather. It is 
sometimes combined with actual mildew, from which it is rather 
difficult to distinguish, even under the microscope, and may be 
caused by fungoid plants, which not only mechanically expel 
the fats by their growth, but probably promote their rancidity 
and the separation of the crystalline fatty acids. It is at most 
only a defect of appearance, and does not in any way injure the 
leather. It was constantly present in calf -kid, from the neatsfoot 
oil used in finishing, and was in this case rather liked by the 
buyers, who for some reason regarded it as a proof of quality. 
A very similar appearance may be caused by the use of solutions 
of barium chloride, alum, or other mineral salts for weighting 
or other purposes ; but is persistent when the leather is held to 
the fire, while the crystallised fatty acids melt and disappear. 
The fatty acids are at once removed by a drop of benzene or 
petroleum spirit, but unaffected by water, while with water- 
soluble salts the reverse occurs. 



470 PRINCIPLES OF LEATHER MANUFACTURE 

The second form of spueing is of a much more troublesome 
character, and makes its first appearance as minute spots or 
pimples of resinous matter raised above the surface of the 
leather, which if removed generally reappear, and which may 
become so bad as to form a sticky resinous coating over the 
whole surface. The exuded matter consists of the oxidised 
products of oxidisable oils, but the cause of its appearance is not 
always easy to explain. The currier generally attributes it to 
adulterated oils, and it must be admitted that some oils almost 
invariably produce it, but it appears occasionally when only the 
purest and absolutely genuine cod oil has been used. It can 
only be produced from drying or semi-drying oils, which include 
all the ordinary fish oils and most of the vegetable seed oils, but 
can never arise from tallow or stearine, from mineral oils or 
vaseline, or from genuine non-drying oils, such as tallow, neats- 
foot, sperm, or mineral oils, nor, probably, from rosin oil. It is 
favoured by causes which promote the oxidation of oils, such as 
moist heat with limited access of air, and by the presence of 
oxygen carriers, such as iron-salts in blacks, and possibly also by 
the presence of free acids. A large amount of free fatty acid in 
the oils themselves is suspicious, not only because the free acids 
oxidise more freely than the neutral fats, but because their 
presence is an evidence of the tendency to rancidity and change 
in the oil. It is also said to be caused by previous mildewing of 
the leather, and certainly often occurs where the grain has been 
rendered porous by bacterial action in the soaks, limes, or bates, 
probably from the greater quantity of oil absorbed by these parts. 
While it is easy to say which oils may possibly spue, there is 
no known chemical test which will foretell whether a given 
sample is likely to do so under ordinary conditions. Eitner ^ 
states that seal oil extracted at a low temperature is very liable 
to spue, but that when heated for a considerable time to a tem- 
perature of 250° to 290° C. it darkens in colour and loses the 
tendency. This is probably true of many other marine oils, 
and may be one cause of the frequent trouble with modern oils, 
many of which, especially the lighter coloured kinds, are extracted 
by steam at a temperature below boihng point. It is very 
probable that one effect of heating to a considerable temperature 
is to dehydrate and separate albuminous or gelatinous matters 
which are present in the fresh oils, and which probably increase 
their tendency to decomposition. Many of these substances 
separate as " foots " from oils during long storing, and such old 
oils are said to be less liable to spue than thpse of recent manu- 
^ Gerber, 1880, p. 243. 



OILS AND FATS IN CURRYING 471 

facture. It is not easy to say why the oil comes in spots to the 
surface instead of remaining inside the leather. Probably 
some expansion of volume occurs with oxidation or by bacterial 
growth, and a pimple of oil once formed on the surface " dries " 
there, but still absorbs oil from below, which dries in turn, and 
so increases the growth. " Spueing " is apt to occur when goods 
have been mildewed. 

If oxidisable oils are used upon leather they " dry " upon the 
fibre if they do not " spue," and if a sufficiency of non-drying 
constituents are not present at the same time the leather will 
ultimately become hard, and may even crack from hardening 
of the fibre. Mineral oils are not liable in this way to form a 
hard coating on the fibre, but as they are slightly volatile, though 
of very high boiling point, they may ultimately evaporate and 
leave the leather insufficiently nourished. From their low 
surface tension they have great powers of capillary penetration, 
as is witnessed by the way that lamp oils " creep " over the 
surface of the lamp, but they have less affinity for water than the 
more oxidisable oils, and probably do not combine so intimately 
with the leather fibre. They are probably better used in com- 
bination with other greases than alone. Their miscibihty with 
water is greatly increased by the addition of sulphonated fish 
oils, and the writer has examined a sample of sulphonated oil 
containing 80 per cent, of mineral oil which emulsified spon- 
taneously when poured into water. The admixture of sohd 
paraffin with stuffing greases has the tendency to make the leather 
feel less greasy and drier than it otherwise would ; and crude 
turpentine and rosin are said to have a still greater effect in this 
direction. 

The water which is required for satisfactory stuffing may in 
some cases be introduced into the stuffing grease as well as into 
the leather. The effect of degras is largely due to the water 
with which it is intimately mixed, and when degras or sod oil is 
deprived of that which it naturally contains, by heating it to 
too high a temperature, either before or after its mixture in a 
stuffing grease, its efficacy is greatly lessened. 

Fal-liquoring (pp. 273, 378) may be considered a special case 
of stuffing, in which the oil is very perfectly emulsified with a 
large quantity of water. In this way very considerable quan- 
tities of oil may be introduced into leather without giving it the 
least greasy feel. Egg-yolk contains about 30 per cent, of an 
oil chemicahy very like ohve, but with a larger proportion of 
palmitin, and may be considered as a very perfect natural fat- 
hquor, containing also some albumen, which serves as " nourish- 



472 PRINCIPLES OF LEATHER MANUFACTURE 

ment " for the leather. If a means of emulsifjdng oHve, lard, or 
tallow oil (with the addition of a little palm oil) with albuminous 
matter as perfectly as in the egg could be discovered, the problem 
of an egg-yolk substitute would in all probability be solved. 
Milk and cream are also natural fat-Hquors. 

Emulsions are suspensions of one liquid in another, in which, 
being freely suspended, the globules always take a spherical 
form. If oil be shaken up with water a system of this sort is 
formed, but quickly separates, the oil floating to the surface, 
but under suitable conditions such mixtures may remain per- 
manent. This is promoted by smallness of the globules. By 
sufficiently vigorous mechanical mixing tolerably permanent 
emulsions may be formed of oil and water alone, and the globules 
of butter in milk can be so broken up by the centrifugal emulsifier 
that cream will no longer rise on standing. Mere shaking is 
not the most efficient way of producing emulsions ; in the centri- 
fugal emulsifier the mixture is forced out through a narrow slit 
between the edges of two discs, so that the oil issues as thin 
sheets, which break up into minute globules, and, on the small 
scale, a very effective appliance consists of a cylindrical vessel 
with a piston covered with fine gauze or perforated metal, 
through which the oil is forced in thin streams, preferably not of 
circular form, which also break up into globules (see p. 91). 

While it is possible to emulsify oil and water simply, the pro- 
cess is much facilitated by the addition of some third substance 
which lowers the interfacial tension between the two liquids, or 
coats the globules when formed so as to prevent their subsequent 
coalescence, and usually both effects are combined. Soaps of 
various sorts are among the most usual and effective additions, 
and probably act in both ways, lowering the tension between 
the liquids, and coating the globules when formed. These soaps 
are not always added directly. All commercial animal and 
vegetable oils contain at least traces of free fatty acids, which 
are saponified by the addition not only of caustic alkalies but 
even of ammonia or alkaline carbonates, and where the oils are 
almost neutral, emulsification is greatly assisted by the addition 
to them of a little commercial oleic acid. With mineral oils, 
which are unsaponifiable, alkalies alone have little saponifying 
effect, and the addition of soaps in some form is necessary. 

Almost any soap will assist saponification, and ordinary 
domestic soaps are often used, but for some purposes are un- 
desirably alkaline. By the cold saponification process described 
on p. 427 perfectly neutral or even superfatted soaps are easily 
made from any saponifiable oil, and if sufficient excess of oil 



OILS AND FATS IN CURRYING ' 473 

be used the soap dissolves at once to a fat-liquor. For many 
purposes of leather manufacture castor oil has been found par- 
ticularly suitable. The quantity of soap used should not be 
too large ; 2 to 4 per cent, is generally sufficient, and larger 
quantities are less effective. Rain or steam water should be used, 
as the presence of the lime and magnesia salts of hard waters 
causes precipitation of insoluble soaps, which do not assist 
emulsification, and may make the leather sticky, or produce other 
ill effects. 

Sulphated oils (p. 441) have come largely into use for pro- 
ducing emulsions. The earliest of these to be used was the 
" Turkey-red " oil of the textile trades, which is sulphated castor, 
and after neutralisation, generally with ammonia, is miscible 
with water in all proportions. Used alone, it produces nice soft 
leathers, though Eitner states that they are apt to harden and 
become tender with time, but generally it is used to emulsify 
other oils. 

Other sulphated oils, however, are now more largely used, 
and especially sulphated fish oils have become an important 
article of commerce. These, as they are put on the market, 
are usually intended for direct application to leather, and 
especially to assist in retaining the colour of sole leather during 
dr5/ing, and they frequently contain a large proportion of mineral 
oil. For use in fat-liquors, however, it would be better to em- 
ploy the sulphated oil alone, and mix the mineral or organic 
oils as desired. On the analysis of sulphated oils much has 
been written lately, and recent volumes of the American 
Leather Chemists' Journal and of that of the S.L.T.C. must be 
consulted. 

In fat-liquoring chrome leathers, soap is an important con- 
stituent, not merely as promoting emulsification, but in fixing 
the basic tanning chrome salt, by forming with it an insoluble 
soap. The same thing may take place with alumina and other 
mineral tannages. It is very important that before goods are 
fat-liquored all the soluble salts should be removed by washing, 
or rendered insoluble by so-called " neutralisation," as otherwise 
they will bleed into the fat -liquor, and precipitate the soap in it 
as an insoluble sticky substance, which will adhere to the surface 
of the leather and render subsequent dyeing and glazing impossible. 

Emulsion may be brought about by other means than those 
just described. Viscous substances such as dextrine, gums, 
and proteids such as albumen render emulsion easier, either by 
lowering the surface tension or by coating the globules, or by 
rendering them less mobile, and so less inclined to coalesce, and 



474 PRINCIPLES OF LEATHER MANUFACTURE 

fine powders such as starch may also coat the globules. The 
starch of the flour used in tawing pastes and in the manufacture 
of " Crown " leather are useful in this way, though they are not 
directly absorbed by the leather, and some mineral powders have 
the same effect, but could not be used in fat4iquors. 



CHAPTER XXVII 

JAPANNED AND ENAMELLED LEATHERS 

Though the information which can be given on the subject of 
this chapter is far from being either so complete or so rehable 
as might be desired, it seems best, in view of the present im- 
portance of the subject, to offer what is possible. For many years 
the manufacture in England was almost entirely in the hands 
of the " master japanners," who held the secret of the boiling 
of enamel, and who would brook little interference either from 
their employers or from the labourers whom they engaged to 
do the actual manual work, and who were quite ignorant of 
chemistry, and worked only by traditional and empirical recipes. 
The natural result was that no progress was made, and that the 
manufacture fell more and more into the hands of Germans and 
Americans, who were not hampered in the same way, and who 
had at the same time the advantage of a far more favourable 
climate. Even yet, though things are better than they were, 
we are hving far too much in a region of trade secrets and em- 
pirical knowledge, which will not be swept away till the problem 
is seriously and openly taken up by the chemists. 

Japanning consists in applying to the leather a coating of oil- 
varnish laid on in successive layers, and usually dried by heat. 
Enamelling is a term applied to precisely the same process when 
the leather is grained or boarded. Japanned is often styled 
" patent " leather, but so far as the writer is aware has never 
been made under any valid patent. Japanning is usually done 
on the flesh side and enamelling on the grain, but flesh -splits 
are often printed and enamelled. 

The first mention of such an idea is in a patent taken in 1799 
by Edmund Prior for painting leather with colours and boiled 
oil and finishing with oil-varnish. No mention is made of 
stoving. In 1805 a patent was taken by Mollersten for the 
application of a mixture of linseed oil, whale oil, horse-grease, 
and lampblack, and details are given of its application which 
are practically the same as those still in use, but a mixture con- 
taining fats and fatty oils could never have produced a usable 
japan, and as patent laws were much less exacting in those 
days, very probably these ingredients were simply given as a 

475 



476 PRINCIPLES OF LEATHER MANUFACTURE 

blind to conceal the secret. Apparently the first japanned leather 
was put on the market in 1822, while enamelled was not made 
till 1837. The next patent was in 1854 ^o^ details of manufac- 
ture, when the varnish ordinarily used was stated to consist of 
" oil, amber, Prussian blue, litharge, white lead, ochre, whiting, 
asphalt, and sometimes copal," and the use of indiarubber is 
claimed. " 

Any ordinary tannage of dressing leather can be japanned, 
but as little fat of any sort can be used in currying, the tannage 
should be soft and mellow for shoe leathers, and, on the other 
hand, for the thicker leathers used for harness the grain must 
be firm and not inchned to " pipe." An essential point is that 
the leather must not contain grease or fatty oil, which causes 
the japan to " throw off " or run unevenly, or dulls its lustre, 
and even traces of grease or oil brought in contact with the 
finished japan make it soft and sticky. The writer has known 
a case where contact with dyed and finished East India sheep- 
skins used as lining ruined a parcel of dress-shoe uppers, though 
they showed no sign of greasiness, but were proved by analysis 
to contain 20 per cent, of sesame oil. The beam work should 
therefore be planned to remove as much natural grease as possible, 
but it has now become almost universal to degrease leather 
with solvents which is intended for japanning. For shoe leathers 
it is most important that the leather should not stretch, as 
otherwise the japan cracks and gapes in lasting, and therefore 
probably the bating or puering should be very moderate, if indeed 
simple deliming is not sufficient, since the more the elastin is 
removed the greater the tendency the leather has to stretch. 
The same points must be considered in chrome leather intended 
for japanning, and especially that of the absence of stretch. 

Currying. — This is much like that of other leathers, and of 
course dependent on the nature of the goods, but here again all 
operations must be directed to remove stretch. The leather must 
be made as level in substance as possible, and for japan must be 
shaved very smooth, but for enamel this is less important, as the 
goods are fluffed or whitened before coating, and generally lightly 
buffed on the grain. The goods are set out thoroughly on a clean 
table, lightly oiled on grain with linseed oil, and either dried out 
or printed when dry enough. A " willow grain " is that usually 
given. 

Dubbing containing any animal fat must be carefully avoided, 
but cod oil may be sparingly used, and on the Continent and in 
America degras is generally employed. Fat-liquoring seems a 
likely way of getting softness with a minimum of suitable oil. 



JAPANNED AND ENAMELLED LEATHERS 477 

but experiment is desirable as to the oils which may be safely- 
employed and their quantity. 

When dry, goods for enamelling are either fluffed or whitened, 
and if too hard they may be softened by sleeking on the flesh 
and lightly grained with cork-board on the flesh side. 

In the United States many large hides are finished in dull- 
coloured enamels for upholstery. A very thin buffing is taken 
off the grain with the band-knife, which is often finished bright 
and smooth in colours for hat-sweats and the like. The up- 
holstery leathers are probably printed before enamelling, and the 
grain is raised afterwards by boarding. 

Japanning. — In the ordinary English process the goods are 
now nailed down on large boards, perhaps 6 feet by 8 feet, which 
fit into the stoves like drawers, and which are covered first with 
thick felt and then with brown paper. Before nailing on, any holes 
in the hides which the japan might pass through are patched with 
brown paper and glue. It is said that a coating of fullers' earth 
is sometimes given at this stage to remove the last traces of 
grease, but the writer has never actually seen it done, and it is 
probably quite unnecessary with benzine degreasing. Some- 
times the leather is darkened by the application of a black or dark 
blue dye before japanning. 

There is no secret about the actual application of the japan, 
which is (or used to be) done by labourers hired by the master 
japanner. The skins first receive a coat of a thick japan, laid on 
with a finely toothed sleeker {raclette, railike), and dried in the 
stove, and this process is sometimes repeated till the coating is 
sufficiently thick, when it is smoothed down with a pumice stone. 
This used to be done by hand with a stone shaped somewhat like 
a horn, but in Germany a rotating stone, carried on a double- 
jointed arm driven by belts and moved over the leather by hand, 
is usually employed, while the leather is supported on a stone 
slab. 

The English drjdng stoves are mostly unventilated, and heated 
by a close gridiron of steam-pipes on the floor to a temperature 
which may be as high as 71° C. (160° F.). German stoves do not 
much exceed 60° C. (140° F.), but a portion of the drying is always 
done in the sun. Americans are said for special purposes to go 
as high as 93° C. (200° F.), in which case the leather is previously 
prepared with a solution of alum and borax, 2 oz. of each to the 
gallon. Drying is there also completed in the sun. 

In the German and American methods the skins are usually 
stretched on frames, often fitted with screws or toggle-joints 
at the corners to admit of their expansion after the skins are 



478 PRINCIPLES OF LEATHER MANUFACTURE 

stretched, instead of on boards. The skins are put on the frames 
damp, and often receive their first coat in this stage so that the 
damp, and the thick nature of the japan may prevent its pene- 
tration, while in others the skins receive a coating of size somewhat 
similar to that used on waxed calf. In Germany the first coats 
are often applied with a sort of spatula on a stone slab, but later 
coats are thinned "with turpentine and applied with a broad 
brush {quelle de monie). In France the preliminary coatings are 
applied on frames, but the finishing is on boards, and the goods 
are exposed to the sun after the coatings are so far dried in the 
stove as not to retain the dust. No exact statement can be made 
of the number of coatings applied, as the practice differs in 
different works and with different classes of goods. 

Preparation of Japans. — The foundation of all japans is linseed 
oil, and with this and Prussian blue alone it is possible to make a 
good japan. 

The purity of the oil is of the first importance, and it is con- 
ceded that Baltic and Belgian seed is the most suitable, most 
others being more or less mixed with other seeds grown at the 
same time. Russian linseed is generally mixed with a certain 
proportion of hemp, but it is doubtful how far this is disadvan- 
tageous, as hemp oil itself makes a very fine lacquer, and is much 
used for this purpose in Russia. The oil is best bought in large 
quantities from a reliable presser and stored in large tanks in a 
warm place, so that clear and well -settled oil is always available 
for boihng. It deposits at first a good deal of " foots," but if 
left in the tanks these gradually disappear or subside to a very 
small bulk, so that even after years the deposit is very small. 
The foots probably consist principally of water and a little 
mucilaginous matter emulsified with the oil. 

Chemically, linseed oil consists for the most part of the gly-, 
cerides of one or two highly unsaturated fatty acids, of which 
linoleic and linolenic are the principal. These have the same 
number of carbon atoms as stearic acid, but the former has two 
double links or pairs of unsaturated bonds and the latter three 
pairs (see p. 432). They are thus capable of absorbing much 
oxygen, and also of polymerisation, or the linking together of 
two or more molecules into more complex ones, thus giving rise 
to resinous products of which the constitution is not well under- 
stood, but which become more soluble in alcohol, and finally 
even in water as the oxidation proceeds. This change, and not 
evaporation, is what constitutes the " drying " of oils. By the 
limited action of oxygen, or more slowly of air, drying oils are 
converted into a sort of jelly, which is much used in the manu- 



JAPANNED AND ENAMELLED LEATHERS 479 

facture of linoleum, and which finally dries to a more or less 
hard varnish. 

Boiling of Oils. — When linseed oil is Jpoiled with certain sub- 
stances called " driers " changes take place which make its 
drying much more rapid. These driers are all either themselves 
oxidising substances or carriers of oxygen, or both, and the effect 
of boiling is much more rapidly attained when air is blown through 
the boiling oil so as to keep the driers fully oxidised. Litharge 
and other lead compounds are the oldest driers, but they darken 
the oil and have some other disadvantages, and manganese com- 
pounds, e.g. peroxide, borate, and resinate, have also come largely 
into use, giving a much paler oil, though not drying quite so 
rapidly. Small quantities of the drier remain dissolved in the 
oil, and lead compounds sometimes cause tarnishing of the film 
through the action of outside causes. The use of manganese 
oxide was first suggested by Faraday for printers' ink. 

In commercial oil-boiling, quantities of 2 or 3 tons at least 
are heated in a steam -jacketed pan to about 116° C. (281° F.) 
by steam at 35 lb. pressure for three or four hours, air being forced 
in at the pan bottom, and the driers (10 lb. per ton and upwards) 
are then added. Large quantities of pungent vapour (acrylic 
and croton aldehydes, etc.) are produced, which are carried to a 
furnace chimney, and the air becomes so far deprived of oxygen 
that a candle will not burn in it. 

Air-blowing is not essential, and laboratory experiments can 
be made on so small a scale as on 50 c.c. of oil heated in a 150 c.c. 
beaker on a sand-bath.^ 

The aim in boiling for paint is to produce a film which will dry 
hard, but for japan it is necessary that the coat should remain 
elastic, or it will crack, i.e. the drying must stop slightly short 
of completion. In the older japans, litharge was generally used as 
a drier and filler, and it seems still to some extent to be employed 
in the United States. In Europe generally only Prussian blue 
(ferric ferrocyanide) is used, which seems at the same time to act 
both as drier and colouring agent. Very few accounts exist in 
literature of the actual preparation and composition of japans, 
and the writer, though at one time he had a good deal to do with 
the manufacture, never had the opportunity of following this part 
of the process, so that the best which can be done is to compare 

1 Cp. F. H. Thorp, Technology Quarterly, 3, pp. 9-17, abstracted J .S.C.I. , 
9, 1890, p. 628. Oil boiled with 0-4 per cent, of litharge at 250° (pre- 
sumably Fahrenheit) for 2\ hours dried on glass in ten hours to a hard 
iilm, while with i per cent, of manganese borate heated for one hour to 
230° it dried in twenty hours. 



48o PRINCIPLES OF LEATHER MANUFACTURE 

and collate the various information at our disposal and try from 
it to evolve a clear and connected scheme. Hennig ^ gives 
considerable detail, of which the following is an abstract. He 
advises very clear and old oil, and purifies (he states, from 
palmitic acid) by exposing to the sun in shallow trays on a lo 
per cent, solution of ferrous sulphate. The clear oil is then 
heated to 50° to 60° C, and mixed with 25 to 30 grm. of red 
fuming nitric acid in large pots holding 30 litres, and provided 
with a series of holes one above another for racking, in which it 
is stirred for four days and allowed to settle, and the clear layer 
gradually drawn off, and filtered in a warm place through a paper 
previously saturated with oil.- This purified oil, which already 
dries pretty quickly, is placed in a boiler of about double the 
capacity of the oil used, to allow for frothing, heated for an hour 
up to about 150° C, and 2^ per cent, of litharge added, ^ well 
stirred in, and the boiling is continued. Towards the end of the 
operation much pungent and combustible vapour is given off, 
which must be carried to a chimney. The oil is now allowed to 
cool and settle for two or three days in upright casks with taps 
4 to 5 cm. from the bottom. " The lowest possible temperature 
promotes the settling of the palmitate of lead." 

The clarified oil is now returned to the boiler, and a saturated 
solution of potassium permanganate is added at the rate of 2 
grm. of the salt per litre of oil, well stirred till the mixture turns 
brown, and heated to 130° till all the water is driven off, when 
7 to 10 per cent, of Prussian blue in the finest powder, sifted 
through silk, is dusted in through a sieve to prevent caking 
and thoroughly mixed, and the temperature is raised with con- 
tinuous stirring till brown vapours are given off, the operation 
lasting two to four hours. When the sample draws in threads 
between the fingers on cooling, and makes a dark brown layer 
without greasy edges on writing paper, the boiling is nearly 
done, but may safely be continued a httle longer. For very 
fine varnishes 3 per cent, of finely powdered gamboge may be 
added, and the boiling continued two hours longer at 150°. 
The lacquer is now allowed to cool slowly, and to settle for a 
week or two in a warm place. It is deep brown by transmitted 
light but deep black by reflected, and is a thick syrupy liquid 

1 Chemiker Zeitung about 1878, reprinted in Der Gerber, 1878, p. 53, 
and in the Leather Manufacturer, 1892, p. 631. 

^ The quantity of litharge used seems unnecessarily large, not more 
than I per cent, being employed in ordinary oil-boiling. No details of 
time are given, or whether a higher temperature is used towards the end of 
the operation. 



JAPANNED AND ENAMELLED LEATHERS 481 

at ordinary temperature, but thin enough to flow from the brush 
when warmed. 1 

In the writer's own experience the oil underwent no previous 
preparation but long settling, and he has no reason to think that 
litharge was used, but a small quantity of " rose spirit " was 
added in the boihng, which was believed to brighten the japan. 
Rose spirit is one of the dyers' mordants, and is probably a 
mixture of' tin chlorides and nitrates. For the first grounding 
japan a " common " Prussian blue of a paler colour, and con- 
taining a large proportion of precipitated alumina, was used, but 
for the later coats only pure Prussian blue of the best quality. 
The alumina no doubt served the purpose of a thickener or 
filler, and the writer's experiments with finely ground kaolin 
were quite promising. The final finishing varnish is the same as 
the japan, but with the addition of copal varnish, which is usually 
that sold by the varnish maker. Villon ^ recommends the follow- 
ing : — I per cent, of finely powdered potassium bichromate is 
added to the oil at 100° C, retaining that temperature for half 
an hour, and then raising to 160°, adding 10 per cent, of Prussian 
blue, and boiling as described, and finally adding 30 per cent, of 
a copal varnish, made by heating 100 parts of boiled oil, 100 
parts of gum copal, 100 parts of shellac, 300 parts of spirit 
of turpentine, and 25 parts of ceresine or mineral wax, in an 
autoclave to 350° for half an hour. Copal can only be dis- 
solved by heating in this way. Copal varnishes are now largely 
superseded for japanned leather by solutions of nitrocellulose, 
which are much more elastic. 

Davis ^ gives considerable detail of American processes of 
japanning, which appears to be gathered from a good source. 
The following is a summary, referring mainly to the large split 
hides used for upholstery, which have been already spoken of. 
The hides are stretched wet on frames, dried by heat with fan 
ventilation, taken off the frames, and softened by a " pin-block " 
machine, in which a head, something like that of a sole-cutting 
press, but studded with short rounded pins, comes down on a 
block with corresponding larger holes or grooves, and finally 
by boarding on a table. Any holes are then patched with paper 
and glue to prevent japan running through, and the hides, again 

^ The addition of permanganate not merely oxidises, but furnishes a 
certain amount of manganese drier. Linolenic acid fully oxidised by 
permanganate yields tetrahydroxystearic acid. The object of the gamboge 
is not obvious, but its use is mentioned by other writers. 

^ Villon, Traite pratique de la Fabrication des Cuirs, Paris, 1889. 

' Davis, The Manufacture of Leather, Philadelphia, 1897. 

31 



482 PRINCIPLES OF LEATHER MANUFACTURE 

stretched on frames, now receive a heavy coat of " daub " or 
first japan, made as follows:— Raw linseed oil is boiled in an 
open pan over fire ^ for about twenty-four hours extending 
over two days, and being stirred continuously during boiling 
with a perforated iron paddle. During the first twelve hours 
it is gradually raised to a temperature of about 300° C, and is 
then allowed to fall to 200° before leaving for the night. On the 
second day it is again raised to 300° to 320° and cooked to a 
jelly, and again cooled to 200°, and the fire completely extin- 
guished, and even the bricks cooled with water, and petroleum- 
naphtha up to about one-third of the volume of the oil is added and 
well stirred in, and this is repeated till about i J times the volume 
of the oil has been added, much of the naphtha being evaporated 
by the heat of the japan. ^ The workmen call this " sweet- 
meats," and it does not form daub till J lb. per gallon of good 
lampblack free from grease has been added and well mixed. The 
daub is applied with a steel slicker about ^ inch thick and io| 
inches long, well worked in, and the excess slicked off. It is then 
thoroughly dried in air or sunshine, and a second coat is given, 
which is dried in the stove at 45° to 50° C. For enamels two coats 
of daub suffice, but for smooth japans a third is generally given. 
Enamels now receive a " slicker-coat " boiled to about the thick- 
ness of treacle, and put on with a smaller and thinner slicker, and 
after drying about twelve hours in the stove are pumiced, and 
receive a coat of enamel varnish, which is made of linseed oil 
and about 2 to 6 oz. per gallon of " Chinese blue," and is boiled 
about eight hours, and thinned with naphtha to the same con- 
sistence as the slicker-coat. (The writer has not been able to 
ascertain definitely the composition of Chinese blue, but it is 
pretty certainly some form of Prussian blue.) The goods are again 

^ Steam boiling is not practicable, from the very high temperatures 
required. Electrical heating would be possible. 

^ This apparently wasteful process is probably based on experience. 
Petroleum-naphtha is not a good solvent for the oxidised and polymerised 
oils of the japan, and if mixed at a lower temperature an emulsion rather 
than a true solution would result, though at a high temperature the two 
liquids are mutually soluble. The natural remedy would be to mix cooler, 
and again heat in an autoclave till solution took place, but with so Volatile 
a liquid as petroleum-naphtha this would produce a very high pressure. 
Probably a paore practical means would be first to dilute the japan with a 
" common solvent " such as turpentine, and then to add the naphtha. 
Common solvents are bodies which dissolve both the constituents to be 
mixed, and if added in sufficient quantity generally lead to their mutual 
solution. Thus a moderate quantity of alcohol will bring about the 
common solution of water and ether. 



JAPANNED AND ENAMELLED LEATHERS 483 

pumiced, and receive a final coat of the same varnish, and are 
dried in the stove at about 60° C, and exposed to the sun and 
air to remove stickiness. 

The smooth japans receive two coats of " black varnish," with 
intermediate pumicing. " Black varnish " is made by boiling 
linseed oil with raw umber (containing ferric hydroxide) for 
eight to ten hours at 260° to 300°, ^ lb. per gallon of which 
is carefully worked in, and it is thinned w:ith naphtha till it will 
flow freely from a brush, with which it is applied. Two coats 
of this are given, with an intermediate pumicing and drying in 
the stove at 75° to 80°, and the last coat, after careful pumicing, 
is dusted first with a dry and then with a wet " sweep " to 
remove every trace of grit and dust. The leather is now ready 
for the finishing varnish, which is very similar to the enamel 
varnish already described, but perhaps boiled a little more, and 
thinned with naphtha till it will flow well from a brush, with 
which it is applied. Three coats are given, and dried for about 
forty-eight hours at 70° to 80° C. 

The most important departures in modern practice from what 
has been described in the foregoing pages have been the use of 
nitrocellulose in japans, and especially in finishing varnishes, 
and the drying by the aid of ultraviolet light in place of exposure 
to- sunshine. Both of these tend to minimise the handicap of 
the English climate, and there are manufacturers in this country 
who are already turning out products which can compete with 
those of the Continent, but naturally full information as to their 
methods is not available to the public. 

The use of nitrocellulose solutions (celluloid) on leather is not 
new. So long ago as 1897 W. F. Reid in conjunction with Mr 
Earle . took out a patent ^ for the enamelling of leather with a 
mixture of a low nitrated cellulose and nitrated castor oil dis- 
solved in acetone. The proportions given in the patent were 
II parts by weight of nitrated oil and 5 parts nitrocellulose. 
About the same period Mr Reid showed the writer a pair of dress 
shoes he was wearing japanned with this material, which were 
bright and much softer and more elastic than ordinary japanned 
leather. Probably a smaller proportion of nitrated oil would 
have given a harder surface. 

The cotton used for the purpose should be similar to that used 
for collodion. In many cases scrap celluloid can be bought which 
merely requires solution. The question of solvents is more 
difficult. For collodion a mixture of alcohol and ether is used, 

1 W. F. Reid and E. J. V. Earle, Eng. Pat. 26677, 1897. Cp. also 
J.S.C.I., 17, 1899, 972. 



484 PRINCIPLES OF LEATHER MANUFACTURE 

and for celluloid varnishes usually acetone and amyl acetate with 
some camphor, but during the war a variety of other solvents 
were used for " dopes " for aeroplane wings, and among others 
trichloroethylene, epichlorohydrin, chloroform, and other organic 
chlorine compounds, but these, though quite effective, have been 
largely abandoned, as their fumes were found very poisonous to 
the work-people. If used at all, very effective ventilation is 
necessary, which should be downward and away from the opera- 
tives, as the vapours are very heavy. A solution once made 
can frequently be diluted without injury with other solvents, 
such as alcohol, benzol, or petroleum spirit. Celluloid is soluble 
in acetone alone, but forms a viscid jelly rather than a true solu- 
tion, and the addition of amyl acetate is necessary to produce a 
bright and coherent coating. 

For some sorts of leather, where a very thin coating is desirable, 
it is probable that celluloid solution alone, with a small proportion 
of nitrated, or even of ordinary, castor oil, and either mixed with 
or applied above pigment colours, might be useful. In the 
United States pigment finishes for light leathers have come largely 
into vogue for covering grain and dyeing defects, and these seem 
sometimes applied with a celluloid medium and by spraying, 
though in other cases water solutions are used in a manner very 
similar to the sole leather finishes described on p. 368. The 
New Explosives Co. Ltd. at Stowmarket supply celluloid 
solutions suitable for such purposes, and could no doubt give 
much useful advice as to their use. They also, I believe, supply 
a small sprayer suitable for experimental purposes. Upholstery 
leathers sprayed with a celluloid varnish would be to a great 
extent waterproofed, so that they could be used for automobiles 
and the like. 

As regards drying by ultraviolet rays, the usual method seems 
to be to carry the skins in frames, suspended from a slow moving 
chain or some similar device, past a battery of mercury lamps. 
These lamps must be blown in quartz, as glass is much less per- 
meable to the ultraviolet rays. The arc is " struck " by allow- 
ing the mercury to flow from one bulb to another. A good 
pattern is made by the Thermal Syndicate Ltd., Wallsend-on- 
Tyne. Some care is necessary in their use, as the light is very 
injurious to the eyes, which should be protected by dark red glass 
in looking at the lamps, and the rays also produce unpleasant 
sunburns, especially on parts of the skin not usually exposed. 
It wiU be remembered that the light is often used by surgeons in 
extirpating malignant skin diseases. 

Ultraviolet rays have a powerful ionising action on the air 



JAPANNED AND ENAMELLED LEATHERS 485 

through which they pass, leading to the production of some 
oxidising substance generally supposed to be ozone, which has 
an injurious action on the leather, and several patents have been 
taken for its removal, especially one by Doerr and Reinhart of 
Worms, which cover the use of ammonia gas ^ in the drying 
chamber. A very competent manufacturer has expressed the 
opinion to the writer that the injurious vapours were not those 
of ozone but of oxides of nitrogen, and that, with sufficient 
ventilation, the difficulty was not a serious one. As the effect 
is produced by radiation and not by temperature, there seems 
no reason why the ventilation should not be as free as that of 
open-air drying, though experiments are desirable as to whether 
it might not be supplemented and quickened by heat applied 
in other ways. 

The present mode of drjdng in hot unventilated stoves seems 
repugnant to common sense, since we have known the great part 
which oxidation takes in the process, and the drying would prob- 
ably be more rapid and complete with better ventilation, even 
if the temperature were somewhat lower. Dr3nng in " tunnels " 
in a current of hot air, the frames being carried on trucks, would 
seem worth trying where goods of uniform sort and size were 
manufactured {cp. p. 558). The process does not seem to be 
wholly one of oxidation, for glass lights and other cooler points 
in the stoves become coated in course of time with dark brown 
sticky matter. 

As regards laboratory experiment much may be done. Small 
japan-boiling experiments may be made as described on p. 479, 
and experirnental samples of japan, either on leather or on glass 
plates, may be dried in an ordinary water-oven regulated by a 
thermostat to the proper temperature, and though the same finish 
and smoothness may not be got as obtained in the works, it is at 
least possible to see whether the japan dries with a smooth bright 
surface, sufficiently hard, and yet not brittle. It would be 
interesting to try comparative experiments in boiling in closed 
flasks to observe the effect of oxidation, and perhaps drying under 
the same conditions might throw light on the question of ventila- 
tion. Different driers and mixtures should be tested. It is 
probable that precipitated hydric ferric oxide in fine powder 
might be substituted fof the expensive Prussian blue, and ferric 

^ The following patents may be referred to : — 
U.S. Pat. 1099378, 1910. 

Junghans, Eng. Pat. 10971, 1912. J.S.C.I., 1912, 694. 
Junghans, Fr. Pat. 443406, 19 13. Coll., 1913, 337. 
Doerr & Reinhardt, D.R.P. 267524. Coll., 1914, 41. 



486 PRINCIPLES OF LEATHER MANUFACTURE 

resinate and borate also suggest themselves. The use of very 
finely levigated ultramarine, especially in the bottom coats to 
correct the brown-black of the japan, seems also worth trial, as 
it is comparatively cheap,- and would probably be unaltered by 
the boiling. When japanning was first introduced Prussian 
blue was probably the cheapest blue known, and the idea was to 
correct the brown of japans made with lampblack. 

Cellulose acetate has been recently much used for "dopes " 
and varnishes, as well as for non-inflammabJe films, and is prob- 
ably applicable in japanning. 



CHAPTER XXVIII 

>. DYES AND DYEING 

Before the discovery of artificial organic dyestuffs th.e only 
colouring materials known to industry were those of mineral 
and direct organic origin, and on this account the dyeing of 
leather was formerly subject to great difficulties and limitations. 

The discovery of the means of artificially preparing an organic 
dyestuff (mauve) by Perkin in 1856 opened up a new field for 
research, and since that time the list of commercial dyes has so 
increased, that there is now scarcely a tint or shade which cannot 
be accurately matched and reproduced by the coal-tar colours. 
These colours are often spoken of as " aniline dyes," owing to 
the fact that many of them, and especially the earlier ones, have 
been derived from aniline, one of the products of coal-tar ; but 
more recently a considerable number of important colours have 
been prepared from other constituents of the tar, and it is there- 
fore more correct to term the whole of the dyes obtained, either 
directly or indirectly, from coal-tar the " coal-tar colours." 

The number of the coal-tar colours is now enormous, and their 
chemistry is far too complex even to touch upon here, and 
possesses a large literature of its own.^ A large number wiU dye 
leather direct, while others must be mordanted like the dyewoods, 
and some are dyed in a colourless form, like indigo, and only 
develop colour on oxidation in the air. The variety of direct- 
dyeing colours is, however, so large, and their use so simple, that 
for ordinary leather dyeing there is little inducement to go beyond 
them. Some colours important for textiles are useless for 
leather, either as requiring too hot a bath or additions such as 
sulphide of sodium, which leather will not stand, and many colours 
much used before the war are no longer available. Mr M. C. 
Lamb, who is a well-known authority on leather dyeing, has, 
however, kindly contributed a list of suitable colours which can 
be obtained at present, but which, it may reasonably be hoped, 
will be extended in the near future. (App. C.) 

The coal-tar colours are generally soluble in water or mixtures 

^ As an introduction to the subject, Artificial' Dyestujfs, their Nature, 
Manufacture, and Uses, by Ramsey and Weston (published by Routledge 
& Sons, Ltd., 3s. 6d.), may be recommended as cheap and as Uttle technical 
as possible. 

487 



488 PRINCIPLES OF LEATHER MANUFACTURE 

of water and alcohol, and the majority of them combine with the 
fibre of the leather without the use of any mordant, so that in 
most cases it is only necessary to apply a solution of the dye 
direct to the leather, though their suitability for the purpose 
varies considerably. A few which are only soluble in oils or 
hydrocarbons are not suitable for leather dyeing, though they 
may sometimes be utilised in conjunction with fats in currying 
or japanning ; and there are also certain colours which are not 
applied to the fibre ready formed, but are developed on it by 
subsequent chemical treatment, and which have only been 
applied to a limited extent to leather. 

A number of the coal-tar dyes, which are produced in the 
crystalline form, have a totally different colour when solid to 
that of their solutions and to the colour they produce when 
dyed. A well-known instance of this is magenta or fuchsine, 
which forms glistening green crystals, while in solution it is a 
brilliant red dye. The colours of the crystals are usually com- 
plementary to those of the solution ; thus several blues have the 
appearance of metallic copper, and violets, such as methyl violet, 
are greenish -yellow, generally with a pronounced metallic lustre. 
This peculiarity is the cause of the defect in dyeing known as 
" bronzing," in which the dye, when applied in too concentrated 
a form, takes a surface shimmer of its complementary colour, 
and is utilised in " bronzed " leathers by appl5dng the dye in a 
concentrated, and often alcoholic, solution. 

The coal-tar colours are mostly either " acid " or " basic." 
The former are the salts of organic colour-acids with inorganic 
bases (generally sodium), and are usually readily soluble in 
water, but frequently do not fix themselves on the fibre till 
the colour-acid is set free by the addition of some stronger acid 
to the bath, and in many cases the free colour-acid is of different 
colour to its salts. The " basic " colours are salts of colour-bases 
(organic bases of the nature of very complicated ammonia deriva- 
tives) with acids (mostly hydrochloric, sulphuric, or acetic). Most 
of these in commercial use are soluble in water, though a few 
require the addition of alcohol. The colour-bases themselves are 
usually insoluble in water, and therefore precipitated by alkahes, 
and in some cases they are also colourless. The basic dyes have 
generally greater intensity of colour than the acid dyes, but large 
classes of them are very fugitive when exposed to light, and in 
strong solution many others are very liable to " bronze," a defect 
which is generally less marked with the acid colours. ^ Acid and 

^ It has been shown by Lamb that many basic colours are much faster 
to Hght on leather than on textiles. 



DYES AND DYEING 489 

basic dyes cannot be employed in mixture in the same dye-bath, 
as they usually precipitate each other. • 

As it is not obvious at first sight whether a given dye is acid 
or basic, a reagent to distinguish them is useful. For this purpose 
a solution of i part of tannic acid and i part of sodium acetate 
in 10 parts (by weight) of water is conveniently employed, which 
gives coloured precipitates with basic dyes, but is not affected 
by acid ones. The fact that basic dyes are precipitated by 
tannins influences their use in leather dyeing, not only as regards 
their fixation on the leather-fibre by the tannin which it contains, 
but as the cause of their precipitation in the dye-bath if great 
care is not taken to avoid the presence of tannins in a soluble 
form, either by very thorough washing, or by fixation of the 
tannin by metallic salts, such as tartar emetic {cp. p. 498). The 
use of the sodium acetate is to combine with the mineral acid 
of the colour-salt, which if left free would prevent complete 
precipitation, by substituting acetic acid, which is much weaker, 
especially in presence of excess of sodium acetate {cp. p. 99). 

In using the terms " acid " and " basic " with regard to dyes, 
it is not to be understood that the dyestuffs as employed are 
acid or alkaline in the sense that vinegar is acid and soda basic, 
but merely that the actual colour-constituent of the salt is in 
the one case of an acid nature, and set free by stronger acids, 
and in the other case is basic, and liberated (and often precipi- 
tated) by stronger alkalies. 

There are several general theories ^ with regard to the fixation 
of colours in dyeing organic fibres, and it is probable that no 
one of them affords a complete explanation in all cases. One 
holds that the action of dyeing is mechanical rather than chemical, 
the colour adhering to the fibre by surface-attraction ; another, 
that an actual chemical compound is formed between the dye 
and the dyed material or one of its constituents ; and a third, 
the " solid solution " theory of Witt, is in a sense intermediate, 
holding that the colouring matter is actually dissolved in the 
dyed fibre. The idea of a solid solution, strange at first, offers 
little difficulty on consideration. The colouring metallic salts 
in tinted glasses exist obviously in solution in the melted glass, 
and can hardly be said to change their condition in this respect 
when the glass becomes solid. Gelatine, indiarubber, and perhaps 
all other colloid bodies, absorb water or other liquids without 

^ Compare theories of dyeing with those with regard to tanning men- 
tioned in Chapter XXXII. There is much in common between the two 
processes, and Zacharias has shown that many dyestuffs have also tanning 
properties. 



490 PRINCIPLES OF LEATHER MANUFACTURE 

losing their solid form, and these liquids may fairly be said to 
be dissolved in the solid. All animal and vegetable fibres are 
in this respect like gelatine, and during the process of dyeing 
are swollen with water. It is quite easy to dye a mass of gelatine 
throughout with most water-soluble dyestuffs. (Compare on 
these points what is said in Chapters IX. and X. on the physical 
chemistry of hide-fibre.) The distinctions between solution and 
molecular surface-attraction on the one hand, and certain forms of 
chemical combination on the other, are not wide ones, and prob- 
ably all three theories are true in different cases, and shade off 
into each other by imperceptible gradations. The subject 
of leather dyeing is, in fact, a very complicated one, since we are 
not dealing with a fibre of uniform composition, but with one 
which has had its structure (both chemical and physical) altered 
by the processes to which it has been subjected during its 
conversion into leather. 

Although, strictly speaking, the constitution of the gelatinous 
fibre of the skin is unknown, we are quite justified in stating ^ 
that, like the amino-acids which are important proximate pro- 
ducts of its decomposition, it contains both acid and basic groups, 
and is therefore capable of attracting both bases and acids. It 
is well known, for instance, that neutral hide-fibre is capable of 
withdrawing sulphuric acid from a decinormal solution with 
such. vigour that the residual liquid is neutral to litmus paper ; 
and it will also absorb caustic alkalies with perhaps equal avidity.^ 

It is thus readily dyed by colouring matter of either basic or 
acid character, and in many cases will even dissociate their salts, 
dyeing the characteristic colour of the free dyestuff, but possibly 
at the same time fixing the liberated base or acid with which 
the colouring matter has been combined. Many tanning pro- 
cesses consist in a somewhat analogous fixation of weak bases 
and acids, and it is therefore to be anticipated that they will 
profoundly modify the colour-fixing properties of the original 
fibre, as indeed proves to be the case. Exactly what the result 
of a particular tanning process in this respect will be is less easy 
to foresee. 

In the ordinary vegetable tanning process the tannins, which 
are of acid nature, are freely fixed by the fibre. It is, therefore, 
not surprising that vegetable tanned leather most readily fixes 
the basic colours, especially as these form insoluble compounds 
with the tannic acids, so that it is quite probable that the dyeing 
is mainly effected by the formation of tannin-colour-lakes on the 
fibre, rather than by actual fixation of the colour-base in com- 

^ Procter, Journ. Soc. Chem. Ind., 1900, p. 23. ^ Cp. Chapter X. 



DYES AND DYEING 491 

bination with the original matter of the skin. It is noteworthy, 
however, that even fully tanned skin has by no means lost its 
attractions for acid colouring matters, many of which will dye it 
even without the presence of free acid, though it is possible that 
the tannic acid performs the function of saturating the alkaline 
base with which the colour acid has been combined. 

It should be pointed out that while the substance of animal 
skin consists practically of gelatinous fibres, it is covered on the 
outer surface with a thin membrane of extreme tenuity, called the 
hyaline or glassy layer (p. 56), which, in the living animal, sepa- 
rates the true skin from the epidermis. This layer, the chemistry 
of which is quite unknown, reacts to colouring matters differently 
from the gelatinous fibres, and probably is less absorbent for 
basic colours, and more so for the coloured anhydrides of the 
tannins, and perhaps for acid colours generally, than is the true 
skin. As a result it colours more darkly in tanning and less so 
in dyeing with basic colours, and as it is extremely liable to 
damage in the preliminary operations of removing hair and lime 
by the tanner, this irregularity of colouring is a serious disad- 
vantage, which is most marked with the basic colours. Small 
quantities of lime left in the skin are also probably important 
causes of irregular dyeing. 

Mordants are chemicals used to enable the fibre to fix dyes 
for which it would not otherwise have sufficient attraction, and 
hence are generally substances which have affinity both for the 
fibre and the dye. Thus cotton, which does not itself attract the 
basic colours, is mordanted for them by a solution of tannin, which 
it attracts, and which, in its turn, attracts and fixes the colours. 
In many cases, however, the function of mordants is more com- 
plex, not merely fixing the dyestuff, but often modifying, or even 
producing, its colour. Thus tannin dyes black on an iron mordant, 
though it is itself colourless. Such mordants may be applied after 
the colouring matter, where the latter has sufficient attraction for 
the fibre to be taken up alone, but does not produce the required 
colour. This process is often called " saddening," as the colour 
is generally darkened. A familiar instance is the use of iron 
solutions to darken or blacken tannin or logwood. There is 
scarcely any distinction in theory between mordants of this 
class and the constituents of dyes which are successively applied 
to the leather in order to produce the colouring matter on the 
fibre. Among these may be mentioned several mineral salts 
which were formerly employed in leather dyeing, though their 
use is now nearly obsolete. Iron salts are easily fixed by leather, 
whether tanned or tawed, and in the former case produce a 



492 PRINCIPLES OF LEATHER MANUFACTURE 

dark colour by action of the tannin. On subsequent treatment 
with a solution of potassium ferrocyanide a deep blue is formed 
(Prussian blue). If copper acetate or ammoniacal solution of 
copper sulphate be substituted for the iron salt a deep red- 
brown ferrocyanide is produced. Yellows are sometimes dyed 
by first treating tanned leathers with lead acetate, which is 
fixed by the tannin, and then with potassium bichromate, by 
which yellow lead chromate is produced. A more important use 
of lead is in the so-called " lead-bleach," which is really a white 
pigment-dyeing with lead sulphate. The tanned leather, after 
washing, is first treated with a solution of lead acetate (usually 
" brown sugar of lead " of about 4 grm. per litre), and subse- 
quently with a dilute sulphuric acid of about 30 grm. of con- 
centrated acid per litre, and then thoroughly washed to free 
it from acid. The process is often used as a preparation for 
dyeing pale shades, as many of the aniline dyes are easily fixed 
on the bleached leather, but is subject to the disadvantage 
attendant on all pigments containing lead of becoming rapidly 
darkened by traces of sulphur or sulphuretted hydrogen, such as 
are constantly contained in lighting gas, or arise from the putre- 
faction of organic matters. The use of acid is also liable to 
cause early decay of the leather if not thoroughly removed. ^ 

A large proportion of the coal-tar colours contain amino- 
groups (NHg groups) which, when treated on the fibre with 
nitrous acid (or an acidified solution of sodium nitrite), become 
" diazotised " (converted into — N : N — groups with elimination 
of OH2). On further treating the diazo-compound with solutions 
of amines or phenols, combination takes place, and new azo- 
colours are formed in or on the fibre, often remarkably fast to 
washing or rubbing. Since these qualities are less important 
in leather than in textiles, and the process is moreover somewhat 
deUcate, and the nitrous acid is apt to affect the leather injuriously, 
these processes have been little used in leather dyeing, and are 
only mentioned here for the sake of completeness. 

The use of the natural polygenetic colours in dyeing leather 
of vegetable tannage, which was once universal, is gradually 
disappearing, except for the production of blacks. Leather 
cannot be very satisfactorily mordanted for these colouring 
matters ; but they have some natural attraction for the leather 
itself,, and are generally dyed first, and their colours afterwards 
developed by metallic mordants such as iron, chrome, tin salts, 
and alum, which act not only on the absorbed dyestufi, but 

^ Barium salts cannot be usefully substituted for those of lead, as they 
have no attraction for the tanned fibre. 



DYES AND DYEING 



493 



frequently on the tannin and colouring matters derived from the 
tanning materials. For black dyeing the use of coal-tar colours, 
either alone or to deepen the colours produced by iron, is 
gradually extending. For suitable colours see App. C. As 
coal-tar blacks are mostly dark violets rather than dead blacks 
their colour may be deepened by the admixture of suitable 
yellows or browns, and this is frequently done by the colour 
manufacturer. Apart from the coal-tar colours, black dyeing 
is generally produced by the action of iron (and chrome), either 
on the tannin of the leather itself or on logwood. As the leather 
is frequently greasy, and the satisfactory formation of a tannin- 
or logwood-lake can only take place in presence of a base to 
absorb the liberated acid of the iron salt, the skins are either 
brushed with, or plunged in, a logwood infusion rendered alkaline 
with soda or ammonia, or the tanned leather receives a pre- 
liminary treatment with weak soda or ammonia solution. As 
such solutions act powerfully on tanned leathers, rendering them 
harsh and tender, great care must be taken to avoid excess. The 
effect of this alkaline treatment is not only to assist the wetting 
of the greasy surface, but to prevent too deep penetration of the 
dye, by causing rapid precipitation of the colour-lake. In recent 
times, however, leathers are sometimes demanded in which the 
colour goes right through, and in this case it might be well to 
reverse the treatment, beginning with a weak solution of a ferrous 
salt, perhaps with addition of sodium acetate or potassium 
tartrate, and finishing with alkaline logwood, as without alkali 
the full colour is not developed. The use of iron salts is not very 
satisfactory in regard to the permanence of the leather ; and 
in this respect it is of great importance that they should not be 
used in excess, and that any strong acids they contain should be 
saturated with permanent bases, and if possible washed out. 
Leather surfaces blacked with iron almost invariably ultimately 
lose their colour, becoming brown if tannins and red if logwood 
has been employed, and at the same time the leather surface 
usually becomes brittle or friable. This is to a large extent 
due to the effect of iron oxides as oxygen carriers. Exposed 
to light they become reduced to the ferrous state, oxidising the 
organic matters with which they are combined, and in the dark 
they re-oxidise, and the process is repeated. It is therefore of 
the first importance that excess of the organic colouring matter 
should be provided, and that the quantity of the iron should 
be as small as possible, and in stable combination. These points 
are greatly neglected in practice, especially where blacking is 
done by the application of iron salts without logwood, when the 



494 PRINCIPLES OF LEATHER MANUFACTURE 

evils mentioned are intensified by the actual removal of part of 
the tannin of the leather, and perhaps by the combination of 
ferric oxide with the skin-fibre itself, forming a brittle iron- 
leather. Treatment with alkaline sumach-, gambler-, or logwood- 
solutions, both before and after the application of the iron, would 
lessen the evil. Iron-logwood blacks are much less permanent 
and fade more rapidly under the influence of light and air than 
iron-tannin blacks. The use of iron blacks on curried leathers 
seems considerably to increase the tendency to " spueing," a 
defect due to oxidation of the oils (see p. 469). Copper salts 
mordant logwood a very dark blue, which is much more stable 
than the iron compound, and hence are often used advantageously 
in mixture with iron salts. In practice, iron blacks are generally 
oiled in finishing, and this renders them more permanent, both by 
protecting the lake from air and by forming iron soaps which 
are stable. The use of actual soaps in blacking and finishing 
is not unknown, and probably deserves more attention. Hard 
soaps of soda and stearic acid^ form an excellent finish where a 
moderate glaze is required, the soap jelly being applied with a 
brush very thinly, allowed to dry thoroughly, and polished with a 
flannel or brush, or glassed. Many acid colours are soluble in 
such soap jellies, which may thus be employed for staining. 
Similar but harder finishes, and capable of being glazed to a 
high polish, are made by dissolving shellac with dilute borax 
or ammonia solutions. ^ Both of these finishes are useful in 
lessening the tendency of iron blacks to smut or rub off, a faihng 
which is due to the precipitation of loose iron lakes on the surface 
instead of in combination with the fibre, and is particularly 
obvious where " inks " or one-solution blacks are employed, or 
where the mordant and the colouring matter solutions are 
allowed to mix on the surface of the leather. Such " inks " are 
generally made with a ferrous salt and logwood or tannin, together 
with some aniline black, and the colour-lake should only be formed 
on oxidation. Chrome is not much employed in blacks with 

1 One of caustic soda in 10 to 15 of water, boiled with 8 of stearic acid 
till clear, cooled to 25° C. and diluted with 400 to 800 water, with constant 
stirring, till a white jelly of suitable consistence is obtained. Somewhat 
similar, but harder,'' preparations may be made with waxes, or fatty acids 
still higher than stearic. Saponified Japan wax would probably produce 
a good glaze. 

2 Five parts of shellac digested warm with 100 water and 3 of ammonia 
fort., or I of borax. If the solution is used as a " seasoning " for glazing, 
the waxy matter which separates on standing should be mixed by shaking 
before use. As ai varnish, a stronger solution should be used and the wax 
skimmed off. 



DYES AND DYEING 495 

vegetable tannages, as it only produces blacks with logwood, the 
chrome compounds of tannins having no colouring value, and 
bichromates being very injurious to the leather if used at all 
freely. 

In dyeing blacks on other than vegetable tannages, however, 
chrome becomes of importance, as logwood is principally em- 
ployed, though sometimes in conjunction with tannin, and often 
with addition of quercitron or fustic, to correct the bluish shade 
of the logwood-chrome or logwood-iron lake. It must not be 
overlooked in practice that if ferrous salts are mixed with 
bichromate solutions the latter are reduced and the iron is 
oxidised to the ferric state. 

In alumed leathers the fixing power of the original hide- 
fibre is much less affected than in vegetable tannages. Whatever 
may be the truth with regard to the latter, there is little doubt 
that physical influences are at least as important as chemical 
ones in the production of mineral tannages.^ The amount of 
the tanning agent absorbed is greatly influenced by the con- 
centration of the solutions, and in ordinary alum tawing much 
of the alumina may again be removed by free washing, especially 
immediately after tawing. In this case the sulphate of potash 
present takes no part in the operation, but both the alumina and 
the acid are absorbed, apparently independently. Alum or 
alumina sulphate alone is incapable of producing any satisfactory 
tannage without the assistance of common salt, without which 
the quantity of alumina absorbed is small, and the fibre becomes 
swollen by the action of the acid. In presence of salt the absorp- 
tion is greater, and the swelhng is prevented. The explanation 
of this is not to be found in the formation of aluminium chloride, 
for though this undoubtedly takes place, it has been shown that 
the action of aluminium chloride without salt is not more satis- 
factory than that of alum. It has long been known that salt 
prevents the swelling action of acids on skin, although it does 
not lessen the absorption of acid ; and the fact is capable of 
explanation on modern osmotic theories {cp. p. 121). The skin so 
treated is found to be converted into leather, but if the salt be 
'washed out the acid is retained by the skin, which returns to the 
state of acid-swollen pelt. It is probable, therefore, that although 
the acid and alumina are absorbed in equivalent proportions to 
each other they are really dissociated and attached to different 
groups in the gelatin molecule, and that the effect of the salt 
is to allow the absorption of the acid without swelling, and, 
osmotically, to increase the dissociating power of the pelt. If, in 
1 Cp. Chapter X. 



496 PRINCIPLES OF LEATHER MANUFACTURE 

place of a normal alumina salt, a basic salt is employed, such as 
may be obtained by partial neutralisation of the sulphuric acid 
with soda, satisfactory tannage may be accomplished without 
salt, a basic compound is absorbed, and the leather is much less 
affected by washing. In the analogous case of chrome tannage 
this basic compound may be still further deprived of its residual 
acid by washing the tanned skin with alkaline solutions, leaving 
a leather which is extremely resistant even to hot water ; and 
a somewhat similar result may be obtained with alumina, though 
with more difficulty, as apparently a very small excess of alkali 
destroys the qualities of the leather {cp. p. 272). 

The results on dyeing are almost what might have been fore- 
seen. While ordinary alumed leather absorbs both acid and 
basic dyes readily, the basic chrome leather has practically lost 
its affinity for the latter. Both chrome and alumina leathers 
readily absorb vegetable tannins, thus supporting the view that 
the acid-fixing groups of the gelatin molecule are still un- 
saturated (tannins are capable of tanning pelt swollen with 
sulphuric acid, and apparently of expelling the acid). In the 
case of chrome leather the effect of re-tanning with tannins is 
greatly to lessen its stretch and, if carried too far, to destroy its 
toughness, but it at once becomes capable of fixing basic dye- 
stuffs. This property is frequently made use of in dyeing, but 
the effect on the leather must not be disregarded where softness 
and stretch are important, as in the case of glove-leathers. 
Polygenetic dyes are, of course, fixed on alum or chrome leathers 
by the alumina or chrome mordant, though apparently the bases 
are not present in the most favourable condition for fixing colours. 
Thus logwood extracted without alkali dyes vegetable-tanned 
, leather yellow, alumed leather violet-blue, and chrome leather 
blackish-violet, and some of the alizarine group dye very well 
on chrome, as its resistance to hot water allows much higher 
temperatures to be used than with most other leathers. The 
tannin contained in dyewoods has the effect of lessening the 
stretch of chrome leathers, but at the same time gives them a 
" fuUer " feel. 

Something should perhaps be said on the dyeing of oil and 
aldehyde leathers, but the subject has as yet been scarcely 
treated scientifically, and our practical knowledge of the subject 
is insufficient to justify theorising (see, however, p. 461). 
Wash-leather which has been bleached with permanganate 
(p. 459) can be dyed pretty readily with most dyes, and if brown 
shades are required it is not necessary to remove the manganic 
oxide formed. 



DYES AND DYEING 497 

Defects in the colour of the finished leather are due to a 
variety of causes, but many are produced by want of cleanli- 
ness and system during the dyeing itself. The greatest care is 
needed in this respect, and in brush-dyeing a different brush 
should be used for each different colour, as it is impossible to 
remove thoroughly all traces of dye by the ordinary methods of 
cleansing. 

Irregular and surface dyeing sometimes occurs owing to too 
rapid fixation of the colours, while in other cases the affinity of 
the dye is too small to allow of reasonable exhaustion of the 
bath. Addition of salts of weak acids, such as potassium 
hydrogen tartrate (tartar), or of those like sodium sulphate, 
which form hydric salts, lessen rapidity of dyeing with acid 
colours, while acids generally increase it, and it is also often 
increased by addition of common salt, which lessens the solu- 
bility of the dye. Weak acids, such as formic, or acid salts, 
such as sodium bisulphate, are generally to be preferred to 
sulphuric acid as an addition to the dye -bath, ^ and if the latter is 
used, great care is desirable in its complete removal. There is 
no doubt that the rapid decay of leather bookbindings and 
upholstery is largely due to the careless use of sulphuric acid in 
" clearing " and dyeing the leather ; ^ and even if it is fully 
removed, it has saturated all bases such as lime, which are 
naturally present in leathers in combination with weak acids, 
and which would otherwise act as some protection from the 
sulphuric acid evolved in burning coal gas. A bath containing 
the salt of some strong base with a weak acid such as sodium 
acetate or potassium tartrate, used after dyeing, would lessen 
both these dangers. 

" Bronzing," the dichroic effect produced by light reflected 
from the surface of many colouring matters, complementary to 
that transmitted by them and reflected by the surface of the 
dyed material, is not peculiar to basic colours, but is generally 
more marked in them than in acid ones. Basic colours, from their 
great affinity for tannins, and consequent rapid dyeing, are apt 
to dye irregularly, and without sufficiently penetrating the leather, 
and if the soluble tannin is not wholly washed out of the skins 
previously to dyeing, it bleeds in the dye-bath, and precipitates 
insoluble tannin lakes, which waste colour and adhere to the 

^ See Report of Committee of Society of Arts on Bookbinding Leathers 
1901. 

- For most acid colours acetic acid has proved too weak to bring out 
their full colouring, but formic acid is quite satisfactory. If sulphuric 
acid is used, a weight equal to that of the dyestuff is sufficient. 

32 



498 PRINCIPLES OF LEATHER MANUFACTURE 

surface of the leather. The inconvenience of basic colours due 
to their too rapid fixation may sometimes be lessened by slight 
acidification of the dye-bath with a weak acid, such as acetic or 
lactic. The acid may be still further " weakened " if desired by 
the addition of its neutral (sodium) salt. The precipitation of 
tannin lakes in the bath may be prevented by previous fixation 
of the tannin with tartar emetic, with titanium potassium 
oxalate or lactate, or some other suitable metallic salt (see p. 492). 
^ The fading of the colours of dyed goods by exposure to light 
is a defect which has been much more investigated in the textile 
industries than in leather manufacture, though in the latter case, 
and especially with regard to bookbinding and furniture leathers, 
it is of even greater importance. It is probable that no colours 
are actually unaffected by strong sunlight, but in many cases the 
action is so slight that it may practically be disregarded, some 
of the coal-tar colours, and especially some of the alizarines, being 
practically permanent, while others, and particularly the aniline 
colours belonging to the triphenylmethane group, such as 
magenta, are so fugitive as to be practically bleached by a week 
of strong sunlight. Chrysoidine and the eosins are also very 
bad in this respect. The fastness of colours to light is a good 
deal influenced by the material on which they are dyed, and but 
little has been published of the results of direct experiments 
on leathers, but Mr M. C. Lamb has carried out a research of 
this nature,^ and the subject is now receiving a good deal of 
attention in other quarters. Experiments are easily made by 
exposing samples to sunlight under glass or in a south window, 
a part of the leather being covered with wood or thick brown 
paper for comparison. The results are often complicated by 
the tendency of all leathers tanned with tannins of the catechol 
group, and especially with turwar {cassia) bark (p. 329), mimosa 
and quebracho, to darken and redden in sunshine, or even by 
exposure to diffused light. Pure sumach tannages are nearly 
free from this defect, and are also much less easily destroyed by 
the action of gas fumes (sulphuric acid) and the other injurious 
influences to which books and furniture are often subjected. ^ 

Want of fastness to friction or rubbing is a defect generally 
more important in textiles than in leather, where it is usually 
prevented by glazings or other finishes applied to the surface, 
but in some cases, and especially in black leather, it is apt to be 
annoying. If suitable colours are used, the defect is generally 

^ See Journ. Soc. Chem. Ind., 1902, pp. 156—158. 

^ Cp. Report of Society of Arts Committee on Bookbinding Leathers, 
1901. 



DYES AND DYEING 499 

due to the precipitation of loose colour on the surface, either by 
the too free use of mordants, or the dyeing of basic colours on 
leathers which have not been sufficiently freed from loose tannin. 
It is also often caused by " flaming," or the application of colour 
mixed with the " seasoning " used in glazing to hide imperfections 
in the dyeing or vary its colour. Colour applied in this way is 
only mechanically fixed on the leather, and is easily removed by 
moisture, staining articles with which it cornes in contact. 

A very similar defect may be caused by incomplete washing 
of the dyed leather, which leaves loose dye from the dye-bath in 
the goods. To avoid it in glove4eathers, where its occurrence 
would be particularly annoying, the natural mordant colours are 
still largely in use, which being precipitated on the fibre in an 
insoluble form by the mordant or " striker " (generally a metallic 
salt) are little liable to come off. Basic colours may be fixed by 
a subsequent treatment with tannin, or by topping with certain 
acid colours such as picric acid. Some few colours, and especially 
Martins or " Manchester " yellow (dinitronaphthol) , are volatile 
at a low temperature, and therefore liable to " mark off " or stain 
any materials with which the dyed fabric, even in a dry state, is 
placed in contact, and some of these colours are also irritating 
to delicate skins. Many tans (gambler, etc.) also " mark off." 

The practical dyeing of leathers varies considerably accord- 
ing to whether they are tanned with vegetable materials, chrome, 
alumina salts, or by chamoising. Vegetable tanned leathers are 
dyed either by hand in the " dye-tray " or in the drum or paddle, 
the two latter methods being now largely employed. The dye- 
tray is a shallow vat, about 10 inches deep, and large enough for 
the goods to be laid flat in it. In the English method one or two 
dozen skins, or even more, are dyed at a time, being turned over 
in the tray by hand,. the undermost pair being drawn out and 
placed on the top (fig. 109). The method is convenient where 
only a small number of skins are to be dyed to one particular 
shade, which is more "easily matched as the goods are always 
under observation, and it has the further advantage that, if 
desired, the grain sides only of the. skins can be coloured, by 
" pairing " or " pleating " them before dyeing. For this purpose 
two skins of equal size are laid together flesh to flesh (pairing), 
or each skin is doubled down the back, flesh side in (pleating), 
and pressed firmly together with a sleeker on the table, when the 
skins adhere so closely that if carefully handled no colour pene- 
trates between them during the dyeing, except a little round the 
edges. This effects considerable economy of dyestuff, as the 
fleshes would absorb a good deal, and for some purposes an un- 



500 PRINCIPLES OF LEATHER MANUFACTURE 

dyed flesh is preferred. In dyeing in the paddle or drum the 
skins are merely placed loose in the dye-liquor, so that the fleshes 
are dyed 'equally with the grain sides. Paddle-dyeing has the 
advantage of effecting a considerable saving of labour as com- 
pared with the dye-tray, in which constant handhng, which often 
lasts an hour or more, is required. It also allows of almost equal 




Fig. 109. — Dyeing in the Tray. 



facility in examining the colour of the skins, which is very im- 
portant when dyeing to shade ; but it is less economical in dye- 
stuff, as not only the flesh sides are dyed but a much larger 
volume of liquor is used, and as the dye-bath can never be entirely 
exhausted, more dye is run away in the used liquor. Drum- 
dyeing is much less expensive in this respect, as the volume of 
liquor may be very small, and from the efficiency of the motion 
the dyeing is very thorough, and penetrates deeply into or through 
the skin, which in many cases is advantageous, but it is difficult 
to dye to exact shade, since the skins can only be examined by 
stopping and opening the drum. Most dyes are more readily 
fixed at high temperatures, and in this respect the drum has an 
advantage over all other methods, as once heated it retains its 
heat with very little loss to the end of the operation, while both 



DYES AND DYEING 501 

in the paddle and the dye-tray the Hquor is rapidly cooled, and 
special methods of maintaining the temperature complicate the 
apparatus, and require great care to avoid overheating. It is 
usually best to work at the highest temperature which the goods 
will safely bear, and this varies to some extent with the class of 
goods, chrome tannages and chamois leather being peculiar in 
standing almost any temperature short of boiling. With vegetable 
tanned leather 50° C. may be taken as a maximum, but 
cold wet skins may safely be introduced rapidly into a liquor 
heated to 60°, as they will cool it sufficiently. 

The Continental method of dyeing in two trays may be men- 
tioned here, as it produces very rapid and even dyeing, with 
considerable economy of dyestuff, and the principle is capable of 
application to other methods where a large number of skins have 
to be dyed to the same colour. As generally carried out two 
trays are employed, each about 4 feet long, 18 inches wide, and 
10 inches or a foot deep, and these are usually made with a slop- 
ing bottom, or propped up in such a way that the dye-liquor all 
runs to the farther side of the tray. A single pair of skins is 
usually dyed at once (in about 6 litres (5 quarts) of liquor for sheep 
and goat). To begin with, the first tray is filled with a very weak 
liquor, and the second with one of about half strength. The 
goods are entered in the first tray, turned a few times, and passed 
into the second ; the liquor in the first is run away, and it is 
refilled with one of the full strength, to which the goods are then 
transferred and dyed to shade. The second tray is much reduced 
in strength by the skins, and now serves as the weak liquor for a 
fresh pair, which in its turn passes into that from which the goods 
have been dyed out, and then into a new liquor, each pair of 
goods thus passing through three baths, of which the last is of 
full strength, and which quickly brings up a full and even colour. 
In the ordinary English method the goods must, for the sake of 
economy of dyestuff, be dyed out in a nearly exhausted bath, 
which is a tedious operation, the last stage of dyeing often taking 
a time far longer than that required to bring the goods nearly up 
to shade, and even then failing to produce a good and full colour. 
This evil may be lessened by adding the dyestuff in several suc- 
cessive portions as the bath becomes exhausted, but cannot be 
altogether avoided with a single tray if any reasonable exhaustion 
of the bath is to be attained. At first sight it seems a very slow 
process to dye the goods in single pairs, but this is to a great 
extent compensated by the rapidity with which they take on 
colour. On the other hand, the solutions must be of considerable 
strength to avoid undue loss of time. In the Continental system 



502 PRINCIPLES OF LEATHER MANUFACTURE 

the dyes, mostly of the coal-tar series, are used as strong solutions, 
and each new dye-bath is made up by filling the tray with a 
definite volume of hot water and adding a measured quantity of 
the dye-solution. 

The re-use of partially exhausted dye-baths is generally limited 
to cases where either single dyes or mixtures of very equal affinity 
for the leather are employed, since where dyes of unequal affinity 
are employed one is more rapidly removed than the other, and 
the shade of the dye-bath is altered. Many dyes sold as single 
colours are really mixtures ,i and alter in shade if successive 
quantities of leather are dyed in their solutions. Basic dyes are 
also apt to be precipitated by traces of tannin washed out of the 
goods, and thus rendered unfit for use a second time. This may 
be avoided by suitable preparation of the goods (see p. 503). 

Much of the success of practical leather dyeing depends on 
proper selection and preparation of the goods. Sound uninjured 
grain is a matter of first importance ; no satisfactory dyeing can 
be expected on skins which through carelessness in soaks, limes, 
or bates are tainted by what is known as " weak grain," caused 
by destruction or injury of the delicate hyaline layer, which 
forms the natural glaze and outer surface of the skin (p. 56). 
For such goods " acid " are to be preferred to " basic " dyes, 
the latter having an especial tendency to dye darker and deeper 
where the grain is imperfect. Goods of different tannages and 
colours should never be dyed together, as they are certain to 
produce different shades in the same dye-bath. Tanned skins 
which have been dried, especially if they have been in stock for 
some time, should be thoroughly softened by soaking in tepid 
water and drumming, a temperature of between 40° and 45° C. 
being most advantageous. Skins, such as calf, of mixed or bark 
tannage, must now be freed from all bloom by scouring with brush 
and if necessary with slate or stone, but great care is requisite to 
avoid injury to the grain. A little borax or other weak alkaline 
solution assists in removing bloom. Fresh sumach-tanned skins 
merely require setting out with a brass or vulcanite sleeker, but 
those which have been long dried often dye more evenly and 
readily if they are re-sumached. 

Dark-coloured tannages, such as Australian bazils, and East 
India sheep and goat tanned with cassia bark, are always im- 

1 Such mixtures may often be detected by putting a drop of their 
solution on blotting-paper, when the dyes form differently coloured rings 
according to their more or less rapid fixation by the paper, or by dusting 
the dry dye very thinly on wet blotting-paper, when each particle produces 
its separate spot. 



DYES AND DYEING 503 

proved by sumaching, and if for light colours, by first stripping 
a portion of the original tan by drumming for a quarter of an 
hour with a weak (^ per cent.) solution of soap powder or borax 
at a temperature of 30° to 35° C. and then passing (after well 
washing in warm water, but with as little exposure as possible 
to the air) through a weak sour of sulphuric acid of i to 2 per 
cent. The acid should now be as thoroughly removed as pos- 
sible by washing in water, and the goods should be sumached. ,, 
The process, and especially the use of sulphuric acid, is always 
deleterious to the skins, and is one of the causes of the early 
decay of coloured bookbindings and furniture leathers. Formic 
or oxalic acid may be substituted for sulphuric with greater safety, 
and the risk of injury from sulphuric, which generally is only 
apparent after the lapse of a considerable time, is a good deal 
lessened by adding to the sumach liquor a small quantity of potas- 
sium tartrate, sodium acetate, or lactate, or some other salt of 
a weak organic acid, which is thus substituted for the much more 
dangerous sulphuric. Except in cases of absolute necessity for 
the production of light shades the use of sulphuric acid should 
not be resorted to, and then only for goods which are not expected 
to possess great permanence. For light shades for bookbinding 
and upholstery good sumach-tanned leathers and organic acids 
only should be employed. Alkaline treatment also demands great 
caution, as excess of strong alkalies is very injurious to the 
leather. Another objectionable method for the preparation of 
leather for very light shades is the use of the lead-bleach 
described on p. 492. 

The sumaching is best done in a drum at a temperature of 
about 40°.^ Lamb advises that i to 2 lb. of sumach per dozen is 
sufficient for calf, and recommends running in this Hquor for 
two or three hours. The skins are then rinsed in water to free 
them from adhering sumach, and set out on a table with a brass 
sleeker, and are now ready for dyeing with " acid " dyestuffs. 
If " basic " dyes are used, thorough washing in several tepid 
waters is necessary to free them from the loose tannin ; and if 
deep colours are to be dyed, it is better, instead of too much 
washing, to fix the tannin, which then serves as a mordant for 
the colour. For blues, blue-greens, or violets this is done with 
a solution of " tartar emetic " (antimony potassium tartrate, of 
5 to 20 grm. per litre according to the amount of tannin to be 
fixed, often with addition of some common salt), which produces 

1 Caution is required in drumming in this and other stages, as the grain 
of tender skins is easily weakened by mechanical friction, leading to 
uneven dyeing. 



504 PRINCIPLES OF LEATHER MANUFACTURE 

no alteration in the colour. For browns, yellows, deep reds, or 
yellow-greens it is advantageous to use titanium-potassium 
lactate or oxalate (2 grm. per litre), which in combination with the 
tannin produces a very permanent yellow coloration on which the 
basic colours dye freely. In many cases the titanium salt is best 
applied after dyeing with one of the dyewoods (Dreher). 

The basic colours usually require simple solution in hot water 
before adding to the dye-bath, and are used in quantities of 
0-5 to 2-5 grm. per litre of dye-bath, according to their colouring 
power, which varies a good deal, and to the depth of shade 
required. The solutions should not be boiled, and some colours 
are injured by too high a temperature. Some colours dissolve 
incompletely, and require filtration through a cotton cloth. As 
basic colours are precipitated by calcium carbonate, it is im- 
portant that "temporary" hard waters should be neutralised 
with acetic or lactic acid till they faintly redden litmus ; and in 
the case of colours which, from their attraction for the leather 
fibre, dye too rapidly, and consequently unevenly, better dyeing 
is often obtained by the use of a small excess of acetic acid, 
which also increases the solubility of the colour. Too much 
acid, however, will prevent the proper exhaustion of the bath. 
Some few colours, now little used, require to be dissolved in the 
first instance in a little methylated spirit ; and the addition of 
spirit will often assist dyeing and staining where the leather is 
slightly greasy, though considerations of cost generally prevent 
its use. Sodium sulphate is not unfrequently added to dyeing 
baths to improve equality of dyeing ; and with some of the 
cotton dyes common salt is used to lessen their solubility and 
facilitate the exhaustion of the dye-bath. 

" Acid " colours usually need the addition of acid to the dye- 
bath to liberate their colour acids, and for this purpose sulphuric 
acid is generally used in weight about equal to that of the colour 
used. Its use is, however, objectionable in this case for the 
same reasons as in bleaching, since it is impossible by mere 
washing to remove it entirely from the leather, which it ulti- 
mately rots when concentrated by exposure to a dry atmosphere 
or high temperature, and it is better to use formic acid to the 
extent of two or three times the weight of the dyestuff . Sodium 
acid sulphate may also be used, but is probably more objection- 
able than an organic acid. Many acid colours, however, dye 
quite satisfactorily from a neutral bath. The acid colours are 
used in somewhat similar quantities to the basic, but are generally 
inferior in colouring power, though they dye more evenly, especi- 
ally on defective grain, and are often more permanent to light. 



DYES AND DYEING 505 

Mention has already been made of the polygenetic or mordant 
dyestuffs which are still used to some extent for dyeing glove- 
leathers, and of which logwood is important in dyeing blacks. 
Fustic and Brazil-wood (peach-wood) are not quite gone out of 
use among old-fashioned dyers, even for dyeing moroccos and 
other coloured leathers of vegetable tannage. Peach-wood 
with a tin mordant (generally a so-caUed " tin spirits," made by 
dissolving tin in mixtures of hydrochloric and nitric acid) was 
formerly much used in dyeing cheap crimsons, but is now quite 
displaced by the azo-scarlets.^ The acid tin-solutions were 
frequently very injurious to the leather. 

The wood - infusion, rendered slightly alkaline with soda, 
ammonia, or, formerly, with stale urine, is usually dyed first on 
the leather, and followed by the mordant " striker " ; ferrous or 
ferric solutions, and potassium bichromate being used for dark 
colours, and tin salts, or sometimes alum, for the brighter ones. 
The mordant is sometimes added to the dye-bath towards the 
end of the operation, but is better used as a separate bath, as it 
is apt to produce a precipitate of colour-lake on the surface of 
the skin, which rubs off on friction. In some cases, and especially 
in black dyeing, the strong infusion of dye-wood and the neces- 
sary " striker " are successively applied by brushing instead of 
in the dye-tray. 

Logwood and Brazil-wood are both Cassalpinias closely allied 
to divi-divi. Logwood is Ccesalpinia Campechianum (see p. 329). 
Its colouring matter is haematoxylin, a substance nearly aUied 
to tannins, and almost colourless ; which on oxidation gives 
hsematin, which dyes directly a yellow-brown, only developing 
other colours by the aid of mordants. Logwood chips are 
extracted by boiling or heating under pressure for some time with 
water ; and as haematin gives dark purplish-red compounds with 
alkalies, soda or stale urine is frequently added under the mis- 
taken belief that it produces a better extraction, but it really 
leads to waste of colouring matter by oxidation. It is best to 
extract with water alone, and add any necessary alkali to the 
infusion before use. One to 2 lb. of wood per gallon is frequently 
employed in making the infusion, and as this proportion of water 
is quite insufficient to properly extract the wood, the residue 
should be boiled with one or more further quantities, which are 
employed in turn for extracting fresh portions of wood. Logwood 

1 Azo-colours are easily reduced and bleached by metallic zinc, and the 
writer once experienced considerable trouble from " galvanised " bolts 
used in the fittings of a copper bath, and zinc if used at all should be pro- 
tected by varnish or japan. 



5o6 PRINCIPLES OF LEATHER MANUFACTURE 

dyes best at high temperatures, and especially in the case of 
chrome leather, with which a temperature of 80° C. may be safely 
used. The presence of a trace of a salt of lime is advantageous, 
and with very soft waters a little lime-water or chalk may be 
added to the logwood liquor. Logwood extracts, being princi- 
pally used for blacks with iron mordants, are frequently adul- 
terated with tanning substances. 

In blacking skins the strong infusion is rendered sHghtly 
alkaline with sodium carbonate or ammonia, and brushed un- 
diluted on the leather. If employed as a bath a somewhat 
weaker infusion is used, and the leather is frequently treated 
first in an alkaline bath to which a small quantity of potassium 
bichromate is often added. The object of the alkali is not only 
to assist in the formation of the colour-lake, by saturating the 
acid set free from the iron salt used as a striker, and thus to 
prevent the colour from penetrating the leather too deeply, but 
at the same time to overcome the resistance to wetting caused 
by grease or oil which the leather may contain. It is possible 
that in some cases sulphonated oil soaps might be used with 
advantage. Alkali must thus be used more freely when stuffed 
leather is to be blacked, but excess should be carefully avoided, 
as it easily renders the leather tender and brittle. The potassium 
bichromate oxidises the hsematoxylin, or the ferrous salt sub- 
sequently applied, and forms a nearly black chrome-logwood 
lake. Bichromates must be used with great caution, as they 
tender the grain, and render it liable to " gape " in subsequent 
stretching. 

The iron solution is generally either of ferrous sulphate of 
perhaps 5 per cent, strength, or commercial " iron-liquor," which 
is a " pyrolignite " or crude acetate of iron, containing catechol 
derivatives and other organic products from the distillation of 
wood, which act advantageously, both as antiseptics and in 
preventing the rapid oxidation which occurs when pure ferrous 
acetate is used. Iron-liquor is generally to be preferred to 
ferrous sulphate (" green vitriol "), as the sulphuric acid of the 
latter, unless completely neutralised by the alkali employed 
in preparation, acts in the end disastrously on the leather. Com- 
mercial iron-liquor is often adulterated with ferrous sulphate, 
which may be detected by its giving a precipitate with barium 
chloride. Great care should be taken not to use iron in excess 
of the logwood or tannin present, as it otherwise takes tannin 
from the leather itself, making it hard and liable to crack, while 
any uncombined iron acts as a carrier of oxygen, giving up its 
oxygen to the colouring matter or tannin with which it is in 



DYES AND DYEING " 507 

contact, and again oxidising from the air, and so causing " spue- 
ing " or oil-oxidation and other evils. The black colour-lakes 
are formed only with ferric iron, and if ferrous salts are used, they 
must be allowed to oxidise in the air after dyeing. 

Good blacks which are more permanent than those with 
logwood may be obtained by merely treating leather containing 
an excess of oak-bark tannin or sumach, first with an alkaline 
solution (not at the most stronger than 2| per cent, of liquid 
ammonia, or 5 per cent, of soda crystals), and then with iron- 
liquor. If it is not certain that the leather contains excess of a 
suitable tannin, a tannin solution must be employed like the 
logwood infusion, or the leather must be sumached. The addi- 
tion of some sumach to logwo.od liquor is often advantageous, 
and a blacker {i.e. less blue) black, especiaUy on alumed leathers, 
is obtained by using a proportion of fustic. Solutions made 
by boiling 10 per cent, of cutch with 5 per cent, of sodium car- 
bonate give good blacks with iron-liquor and do not make the 
leather tender, and they can be used in mixture with logwood. 
Many commercial logwood extracts contain chestnut-wood 
extract as an adulterant. 

Instead of dyeing in the bath it is very common, especially 
for the cheaper leathers, such as linings and coloured leathers of 
the commoner sort, to apply the colour by brushing (commonly 
called " staining "). Many colours, however, which dye well with 
time and warmth are inapplicable in this way, and only those 
should be used which have a strong attraction for the leather, and 
hence go on well in the cold. If " acid " colours are employed, it 
is essential to select those which can be used in neutral solution, 
or at most with addition of some mild organic acid such as formic 
or acetic, since, as the leather is not washed after staining, the 
sulphuric acid would remain in it, and would ultimately destroy it. 
Where leathers have a hard and repellent surface the addition of 
a little methylated spirit to the dye is often very useful. The 
colours are used in solutions of from ^ to i per cent., which 
should be quite clear and free from sediment. Difficultly soluble 
colours must be used in weak solution, or the dye kept warm 
while in use. Dye-solutions will not generally keep for any great 
length of time without change. Acid colours are sometimes 
employed dissolved in diluted " stearine glaze " (p. 494), and the 
use of sulphonated oil emulsions is worth trjdng. 

Before staining, the leather must be carefully " set out," or 
otherwise made as smooth as possible, and the staining is generally 
done after most of the other operations of curr5^ng or dressing 
have been completed. Staining is best begun with the leather 



5o8 PRINCIPLES OF LEATHER MANUFACTURE 

in a slightly damp or " sammed " condition, and the colour is 
applied evenly with a softish brush in two or three coats, the 
leather being slightly dried after each. As a rule, the more coats 
are applied the more even is the work ; but to save cost of labour 
it is common on cheap goods to be content with two, of which 
the first is given, preferably with a weaker solution, to the dry 
leather. Where the leather is " weak-grained " it is sometimes 
advantageous to size it first with a weak solution of gelatine, gum 
tragacanth, or linseed mucilage, and similar solutions are often 
used to fix the colour and give a higher gloss. A weak solution 
of the stearine glaze mentioned on p. 494 is sometimes employed 
as .a vehicle for the acid colours. Acid yellows and browns may 
also be dissolved in the undiluted glaze where only a pale colour 
is required, or to heighten the colour of leather already stained. 
A list of suitable colours for staining is given in Appendix C, 
p. 640. 

It rarely happens in leather dyeing that the required colour 
can be given by the application of a single dye, most of the 
shades now required being produced by mixtures. It is, there- 
fore, necessary to say a few words on the theory of colour 
combinations. 

White light is of course composed of a mixture of all the 
spectrum colours, and can be separated into them by the prism. 
It is probable, however, that the eye is only capable of three 
distinct colour-sensations, and that all the colours we perceive are 
represented by the excitement of these in different proportions, 
the actual colour-sensations being red, blue-green, and violet.^ 
If we interpose a piece of yellow glass between the eye and white 
light the violet and blue are absorbed, and the remaining red 
and green rays combine to produce the sensation of yellow. If 

^ The subject of colour is too complicated to be adequately treated 
here, and for fuller information readers are referred to Abney's Colour 
Measurement and Mixture, S.P.C.K., London, 1891. This is now, un- 
fortunately, out of print, but there are several more recent books, among 
which may be mentioned Luckiesh, Constable, 191 5. It may, however, 
be pointed out that, while the true primary colour-sensations are un- 
questionably red, blue-green, and violet, and by mixture of light of 
these colours all other colours, including white, can be produced, the 
primary pigments or dyes are red, yellow, and blue, the effect being pro- 
duced in the former case by the addition of colours, and in the latter by 
their subtraction. Much useful information can be obtained by the use 
of a pocket spectroscope, or even of an ordinary prism fitted into a box 
[e.g. a cigar-box) with a narrow cardboard "slit " parallel with the prism 
at the opposite end. Dyed materials are examined in a good daylight, 
dye-solutions in tpst-tubes by daylight or in front of a Welsbach or electric 
light. The stronger the light the narrower the slit which should be used. 



DYES AND DYEING 509 

pure blue glass is used the red is absorbed, and we have blue 
as the result of the remaining mixture of green, blue, and violet. 
Red glass absorbs the whole of the green and greenish-blue, allow- 
ing red, yellow and much of the violet to pass. Thus if we com- 
bine blue and yellow glass only the green is allowed to pass, and 
similarly with red and blue glass green and blue is cut out, and 
only the violet remains. Thus red, yellow, and blue are frequently 
called the primary colours, and by combining all three in equal 
proportions all colours are cut out, and black or grey results. 
The blue and violet which are stopped by yellow glass are those 
coloufs which would produce the sensation of violet-blue, and 
hence the latter is called the " complementary colour " of yellow, 
and so on with the rest. It will be noted that all the colours of 
coloured objects are produced by absorption of a part of the light, 
and therefore coloured bodies are always darker than white ones, 
and where a colour is mixed with its complementary in suitable 
proportion, all colours are absorbed and black or grey is produced. 
Colours which are made by mixing two primary colours are 
generally called " secondary " ; while the duller tints made by 
the addition to these of black, or of a complementary colour 
which produces black, are called " tertiary." Any primary colour 
is complementary to the secondary colour produced by mixing 
the other two primaries, and vice versa. The following tabular 
arrangement shows at once the effect of colour mixing: — 

Primary. Secondary. Tertiary. 

Red \ . Orange with Black Brown. 

YeUow I 

I . Green ,, Olive, Sage. 



Blue ^ 
Red 



Purple (Violet) „ Puce, Maroon. 



Theoretically, any colour may be obtained by mixture of the 
primaries, and that this is possible to a great extent is shown in 
the success of modern " three-colour " printing, by which pictures 
are obtained in natural colours by the use of three primaries only ; 
but in practice few colours are quite pure, and if two very different 
colours are mixed, it is difficult to avoid the production of 
tertiaries. The most brilliant colours are generally produced 
by dyeing with the nearest colour which can be obtained to that 
required, and shading with another which is near, but on the 
other side of the desired tint. 

Thus if we want to produce bright shades in dyeing we must 
avoid the introduction of complementary colours. A bluish-red 



510 PRINCIPLES OF LEATHER MANUFACTURE 

mixed with a reddish-blue will produce a bright shade of violet, 
but if we mix an orange-red with a greenish-blue we introduce 
yellow into the mixture, and obtain a dull maroon or puce, 
according to the proportion of the other colours. In a similar 
way, the introduction of a blue dye will dull a bright orange to 
a brown, and a little of a yellow dye will dull a bright purple to 
a maroon. This fact is frequently used in producing the quiet 
shades of colour often required from the most brilhant dyes. If 
to a bright orange we add black, or a blue dye which as its com- 
plementary produces black, we convert it into a brown. If 
instead of blue we use green for dulling we give the brown a 
yellower shade, since the green produces black at the expense of 
the red of the orange. Violet similarly used gives a redder brown, 
since it produces black by combination with the yellow. This 
shading, if small in amount, is frequently done by direct mixture 
of a suitable dye, but if considerable, it is generally better to top 
one colour with another. Thus a blue topped with a powerful 
orange will produce a Havanna brown. For dark colours it 
is frequently convenient to produce a dark ground with some 
cheap dye, such as logwood and iron or chrome, and to top it 
with a bright shade of the colour required. In this way cheap 
dark blues and greens can be easily produced. For reds and 
browns, mixtures of logwood and Brazil-wood, or Brazil-wood and 
fustic, may be used, topped with coal-tar colours. Tanning 
materials, such as quebracho and mangrove extracts, which give 
browns with bichromate, are also employed on cheap goods. It 
is also frequently wise to dye with a basic colour and top with 
an acid one, or vice versa, as in many cases the one fixes and 
combines with the other, and an increase of fastness is obtained. 

Morocco and many other coloured leathers are finished by 
damping the surface of the dried leather with a very dilute 
" seasoning " of water, milk, and blood or albumen, allowing the 
leather to become quite or nearly dry, and polishing by friction 
under a cylinder of agate,^ glass, or wood in the glazing machine. 
Many leathers are also grained by printing from engraved or 
electrotype rollers, or by " boarding," or a combination of the 
two. " Boarding " consists in pushing forward a fold in the 
leather on a table with a flat board roughed underneath, or lined 
with cork, in a way which is difficult to describe, but which in 
skilful hands wrinkles or " grains " the skin in a regular pattern. 

The colour of a dyed skin is much altered by finishing, and 

^ Agate is best for a final glaze, but hard wood (box or lignum vitse) gets 
better to the bottom of the grain. Glass is less apt to " seize " in the 
machine if its high polish is taken off by fine emery or carborundum paper. 



DYES AND DYEING 511 

especially by glazing, which always darkens and . enriches the 
colour. In dyeing to pattern it is useful to glaze a little bit of 
the rapidly dried skin by friction with a smooth piece of hard 
wood or bone {e.g. the handle of a tooth-brush) for comparison, 
and a portion of the pattern may also be wetted for comparison 
with the wet skin. Colours which look full and even in the dye- 
bath often go down in a most disappointing manner on drying, 
though to some extent they regain intensity on finishing. 

In comparing the dyeing value of colours, the most practical 
way is to make actual dyeing trials with equal or known quantities 
of the colours and of water. Such trials may be made, either 
by " turning " the samples in photographic porcelain trays, kept 
warm in a water-bath (a " dripping tin " may be used for the 
purpose, the trays being supported a little above the bottom on 
tin supports soldered to the tin), or the leather may be hung from 
glass rods, by hooks of copper wire, in glass vessels (square 
battery jars), also placed in a water-bath. The leather samples 
should be of equal surface in every case ; for suspension, pieces of 
" skiver " (sheep-grain) of 8 by 4 inches or 20 by 10 cm. are very 
convenient. These may either be " pleated " or suspended by 
the two ends grain side out, with a short glass rod to weight the 
fold and keep them flat. The weight of colour used for a sample 
8 inches by 4 inches multiplied by 54 times the area of a single 
skin in feet will give approximately the weight of colour needed 
per dozen, which is, however, a good deal influenced by the mode 
of dyeing and the quantity of water used. 

In dyeing on the large scale, iron, zinc, and even copper are 
to be avoided, the latter acting very injuriously on many colours, 
and on the whole wooden vessels are to be preferred. Though 
these become deeply dyed, they become very hard, and if well 
washed with hot water, and occasionally with dilute acid, they 
may be cleansed so as to give up no colour in subsequent dyeing 
operations, though of course it is not desirable, if it can be avoided, 
to use the same vessel for very different colours. Probably slate, 
jointed with red lead or pitch, is the ideal material for dye-vats 
and paddles. 

It is 'Useless in the present state of the dye-trade to repeat 
former Hsts of leather-dyeing colours, but Mr M. C. Lamb has 
kindly furnished in Appendix C a hst of the best colours now 
available, which of course must be considered as somewhat 
temporary, and which will probably be considerably extended in 
the near future. Zinc rapidly bleaches many colours, especially 
while wet and shghtly acid, and discharge-patterns may often be 
produced b}^ pressing the wet leather on perforated zinc plates. 



CHAPTER XXIX 

EVAPORATION, HEATING, AND DRYING 

Questions of evaporation, whether for raising steam or for the 
concentration of tanning extracts and other solutions, are of 
considerable importance in the tanning industry, and as the 
same natural laws which apply to these equally govern the 
drying of leather, it is convenient to study the theory of the 
whole subject in one chapter, rather than to divide it and place 
each part in a different portion of the book. 

The modern conception of evaporation and vapour pressures 
has been described on p. 86, but it will be necessary to re- 
capitulate a little. It is a well-known fact that most liquids, 
if left exposed in an open vessel, gradually disappear by evapora- 
tion from the surface into the air, even at ordinary temperatures. 
If the vessel is heated sufficiently the hquid " boils," that is, 
bubbles of vapour are formed within it and escape, and the 
evaporation is therefore much more rapid. To avoid complica- 
tion, let us first imagine a liquid sealed in a glass flask, which con- 
tains no air, but which is only partially filled by the liquid. It 
has been pointed out that the motion of heat by which the 
molecules of the liquid are agitated enables some of them to 
break away from the attraction by which liquid particles are held 
together and pass into the form of gas or vapour, which will fill 
the empty part of the flask. This evaporation will, however, 
soon reach a limit, since the vapour cannot escape from the flask. 
The fl5nng molecules of vapour produce pressure by striking the 
walls of the flask, while a proportion of them will strike the surface 
of the liquid, and again be caught and retained by its attraction ; 
and as the pressure rises, the number of these necessarily in- 
creases till a point is reached when as many fall back and are 
retained (or " condensed ") as those which evaporate, and the 
pressure will then remain constant. The amount of the pressure 
will vary with the nature of the liquid, and will be the greater 
the more volatile it is, or, in other words, the less the power of 
its internal attraction. It will also increase with rising tempera- 
ture, which, by increasing the velocity of motion of the molecules, 
renders their escape from the liquid easier, and their recapture 
more difficult. It will not be at all affected by the volume of 

512 



EVAPORATION, HEATING, AND DRYING 513 

vapour or the size of the flask, but so long as any hquid is present 
it will depend merely upon the nature of the liquid and the 
temperature. If the flask is large, more of the liquid will eva- 
porate till the same pressure is reached. If at the outset the 
flask is not empty, but fiUed with air, it will make no difference 
to the pressure or quantity of the vapour in it, which will be 
added to that of the air, whatever that may be. If the sealing 
of the flask is broken so that it is open to the atmosphere air 
and vapour will escape, or air will pass in, till the total pressure 
is equal to the atmospheric pressure outside (about 15 lb. per 
square inch). As, however, the vapour in the flask is always 
renewed by evaporation, so that the full vapour-pressure of the 
liquid is maintained, the " partial " pressure (as it is called) of 
the air in the flask will be less than that of the outer atmosphere 
by the amount of the vapour-pressure, which makes up the 
difference. Once this balance is attained, evaporation will go 
on very slowly in the flask, as it can only replace the small 
quantity of vapour which escapes. If, however, the vapour is 
removed by blowing fresh air into the flask, it will rapidly be 
replaced in the old proportion by fresh evaporation. Thus 
goods in a close room will dry only very slowly, even if the 
temperature is high, unless the moistened air is replaced by 
drier air from the outside by some effective system of ventila- 
tion. In absence of this, evaporation only becomes rapid when 
the temperature of the liquid is raised to its " boiling point," 
that is, when the vapour-pressure becomes slightly in excess of 
that of the atmosphere, so that the freshly formed vapour can 
push out that already in the flask or chamber into the outer 
air, and at the same time bubbles can be formed in the interior 
of the liquid by the escaping vapour. As the vapour-pressure 
of a liquid rises continuously with increasing temperature, and 
its boiling point is defined as that temperature at which it is 
equal in pressure to the air (or vapour) in contact with it, it is 
evident that the boihng point must entirely depend on the pres- 
sure. Thus the boihng point of water in a boiler at a pressure 
of 55 lb. per square inch above the atmosphere is 150° C, and in 
a partial vacuum equal to 5-8 inches of barometric pressure is 
only 60° C, a fact which is made use of in the concentration 
of extracts and other liquids at a low temperature in the vacuum- 
pan and in the Nance system of tannings (p. 572) . (Atmospheric 
pressure is taken at 30 inches or 760 milhmeters of the barometer, 
or 14-7 lb. per inch, or 1-033 kilos per square centimeter.^) 

1 The German spelling of metrical units has been allowed to remain 
from the first edition, both to avoid extensive correction and because 

33 



514 PRINCIPLES OF LEATHER MANUFACTURE 

If a piece of iron is placed over a powerful gas-burner it will 
go on getting hotter till its temperature is nearly or quite equal 
to that of the gas-flame. On the other hand, a pan of water, in 
the same condition, once it has reached its boiling point, becomes 
no hotter till all the water is evaporated. It is evident that the 
whole available heat or energy of the gas-flame is consumed in 
converting the water into steam. We might convert a propor- 
tion of this energy into mechanical work by using the steam 
in a steam engine ; but even without this, work is actually 
being done by the escaping steam in raising the weight of the 
atmosphere, and in overcoming the attractive force which holds 
the particles of water together in the liquid form. It is of course 
known to everyone that though energy may change its form, 
as from heat to work, it cannot be destroyed, diminished, or 
increased ; and therefore the whole of the work performed in 
converting the water into steam is again recovered as heat when 
the steam is condensed. In this connection a clear distinction 
must be 'made between quantity of heat and temperature, which 
in popular language are often confused. It is, for instance, 
obvious that if we mix a pound of water at boiling temperature 
with another pound at freezing point the temperature is altered 
to 50° C, but the total quantity of heat is unchanged. It is equally 
clear that^no change in quantity of heat takes place when i lb. 
of mercury at 100° is mixed with i lb. of water at 0°, though in 
this case, owing to the small capacity of mercury for heat, the 
common temperature would only be raised to about 3°. We 
must therefore have some measure of quantity of heat apart 
from the mere direct indications of the thermometer, and that 
most generally used is the quantity of heat required to raise i kilo 
of water 10° C. (kilogram-calorie) .^ In England the heat required 
to raise i lb. of water 1° F. is also in use as a unit. The k.-calorie 
is equal to 3-97 (very approximately 4) lb. x F. or B.T.U.^ 
For our purpose it may be taken that 100 k. -calories of heat are 
required to raise i kilo or litre of water from freezing to boiling 

it is phonetic English, and it seems unnecessary to adhere to a French spell- 
ing where we do not adopt the French pronunciation {cp. Appendix A). 

^ A gram-calorie of one-thousandth part of the above is also in use for 
some scientific purposes, but the kilogram-calorie only is used m the 
following pages. 

^ This unit, commonly known as the B.T.U., or British thermal unit, 
i.e. the quantity of heat required to raise i lb. of water 1° F., is much 
less convenient in calculation than the calorie, and it is a pity that it has 
been legalised as the unit of heating power by which gas must be sold, 
since it will involve an additional change when we adopt decimal weights 
and measures. 



EVAPORATION, HEATING, AND DRYING 515 

temperature. If, however, the water is actually frozen we 
require 80 k. -calories merely to melt the kilogram of ice without 
perceptibly raising its temperature, and when the water is raised 
to 100°, 536 calories of heat are still necessary merely to convert 
it into steam at the same temperature. To melt i Ib^ of ice 
requires 144 B.T.U.,^ to raise it to boiling point 180 more, and 
to evaporate it 965 additional. The quantity of heat required 
for actual evaporation varies a little at different temperatures, 
being somewhat larger at lower temperatures, but the total heat 
required to raise water from the freezing point and convert it 
into steam at any pressure is nearly constant, being 635 calories, 
at atmospheric pressure, and only about 650 calories, or 1180 
B.T.U., at 50 lb. per square inch. The quantity of heat evolved 
by the combustion of i lb. of good coal is 13,000 to 15,000 B.T.U., 
or of I kilo, 7200 to 8300 k. -calories, but in raising steam in a 
good boiler, coal will only evaporate 10 times its weight of water 
at 100° (5360 calories or 9650 B.T.U.), the remaining heat being 
lost. One horse-power (33,000 foot-pounds per minute)^ in the 
best engines requires about i| lb. of coal or 15 lb. of steam per 
hour, but in those of worse construction may run up to many 
times that amount. As, even theoretically, not 20 per cent, of 
the total heat can be converted into mechanical work in a " per- 
fect " engine working at 75 lb. pressure, it is often economical 
to use waste steam for heating or evaporation, and where 
this can be done, the additional cost of the mechanical power 
is very small. The " internal combustion " (gas) engine is 
decidedly more economical in heat used, and the Diesel oil engine 
is stiU more so, since they work at much higher temperatures. 

In evaporating liquids in the open pan 536 calories is required 
to evaporate i kilo of water already raised to boiling temperature, 
and a larger amount for salt-solutions, and it makes compara- 
tively little difference whether this is done at 100° or at a higher 
or lower temperature. Where, however, evaporation is done 
in vacuo considerable economy can be effected by what are known 
as multiple " effects," in which the steam from one vacuum-pan 
is employed to boil a second under a reduced pressure, and 
consequently boiling at a lower temperature. This principle 
can be practically apphed to as many as five or six successive 
" effects," the weaker liquor being usually evaporated at the 

^ It might seem at first sight that as i k.-calorie = 3'97 B.T.U., 3i7'6 of 
the latter would be needed to melt i lb. of ice, but it must be remembered 
that I lb. is only 0456 kg., and multiplying by this we get I44. 

2 This is equal to 76-04 kilogrammeters per second, but the metrical horse- 
power is only taken at 75 kilogrammeters in France and Germany. 



5i6 PRINCIPLES OF LEATHER MANUFACTURE 

highest temperature and lowest vacuum in the first " effect " 
by the exhaust steam of the engine used for the vacuum pumps, 
while the steam from the first effect heats that of the next higher 
concentration, and so on. In the Yaryan evaporator (p. 408) 
the boiling liquid is sprayed through coil-tubes, thus exposing an 
enormous surface to evaporation, and the whole concentration 
of any given portion of liquid takes place as it passes through the 
apparatus, which does not, even in multiple effects, occupy more 
than four or five minutes, and without the temperature of the 
liquid ever rising above 60° or 70° C. In the case of liquids like 
sugar- and tannin-solutions, which are liable to chemical change 
from continued heating, the shortness of the time is a very great 
advantage. The number of effects which it is desirable to use 
depends greatly on the cost of fuel as compared to the largely in- 
creased cost of the apparatus. One lb. of coal employed in raising 
steam will evaporate 8| lb. in a single-effect Yaryan, 16 lb. in a 
double-effect, 23^ lb. in a triple, 3o|- lb. in a quadruple, and 37 lb. 
in a quintuple-effect apparatus,^ but as a rule more than three 
or four effects are not advisable (see also p. 407). 

Where liquids are evaporated in the open air at temperatures 
below boiling it is advisable by some means to spread the liquid 
in a thin film, so as to expose a large surface, which must be 
continuously removed by agitation so as to prevent the forma- 
tion of a skin. An apparatus for this purpose is the Chenalier 
evaporator (fig. no), which consists of steam-heated copper discs 
rotating in a trough containing the liquid, which is taken up by 
buckets attached to the rims of the discs and poured over their 
heated surfaces. In other forms the liquid is allowed to trickle 
over steam-heated pipes or corrugated plates. Such evaporators 
should be placed in a current of air so as to rapidly carry off 
the vapour formed. Their use is very objectionable for liquids 
like tannin-liquors, which are injured by oxidation, and they are 
not nearly so economical as vacuum-pans. 

The drying of leather depends on the same laws as the evapora- 
tion of liquids, but demands special consideration from its very 
different conditions of temperature and supply of heat. It is 
important to remember that evaporation cannot go on unless 
the vapour-pressure of the liquid to be evaporated is higher 
than that of the vapour in contact with it, and that air-pressure 

^ Of course a similar economy without vacuum may be effected with 
solutions not injured by temperature by working the first " effect " at 
high pressure and temperature and coming down through successive 
effects to boiling point and atmospheric pressure, though this is not so 
commonly done. 



EVAPORATION, HEATING, AND DRYING 517 

does not prevent evaporation, so that if we sweep away the 
stagnant vapour with dry air, evaporation will go on as quickly 
as in vactto, except that the liquid cannot boil. We must also 
bear in mind that evaporation consumes quite as much heat at 
low temperatures as in a steam boiler, and that this heat must 




Fig. no. — Chenalier Evaporator and Glue Coolers. 

generally come from the surrounding air, the temperature of 
which it reduces. 

The rapidity of evaporation, and the quantity of moisture 
which can be taken up by a given volume of air, depends on the 
vapour-pressure, which increases with temperature. The relation 
between the two, and the weight of water in grams per cubic 
meter which can be dissolved in dry air, is given in the following 
table. (Grams per cubic meter is practically equivalent to 
ounces per 1000 cubic feet. Vapour-pressure is given in milli- 
meters of mercury of the barometer, p. 513.) 100 mm.=:3-94 
inches. 

Vapour Pressure of Water 

Temperature, ° C. — lo —5 o 5 10 15 20 25 30 35 40 

°F. 14 23 32 41 50 59 68 77 86 95 104 

Pressure, mm. 2-2 3-2 4-6 6-5 g-i 12-7 17-4 23-5 31-5 41-9 54-9 

Grams per cub. m. 2-4 3-4 4-9 6-8 9-3 12-8 17-2 22-8 30-1 39-2 



5i8 PRINCIPLES OF LEATHER MANUFACTURE 

Air is practically never dry, and in damp weather is frequently 
saturated with moisture to the full extent corresponding to its 
temperature. In England the average quantity of moisture 
contained in the air throughout the year is 82 per cent, of the 
total possible, and even in the driest summer weather it is never 
less than 58 per cent. So long as the water is in the form of 
vapour the air remains quite clear and does not feel damp ; in 
fogs the air is not only saturated with moisture, but contains 
small liquid particles floating in it.^ Of course when the air is 
really saturated with moisture it has no drjdng power whatever. 

As is evident from the table, the amount of water which can 
be dissolved in a given volume of air rapidly increases with 
temperature. Air at 0° C. is only capable of containing 4-9 
grams per cubic meter, or not much more than 20 per cent, of 
what it can contain at 25° C. It hence rapidly increases in 
drying power as it is warmed, and consequently the air in a 
warm well-ventilated drying room in winter is generally much 
drier, and has greater capacity for absorbing moisture than the 
open air in the driest summer weather. This is the principal 
cause of the tendency to harsh and irregular drying by the use 
of artificial heat, and may be remedied by a proper circulation 
of the air by a fan without too frequent change with the colder 
air outside. On the other hand, the use of a little artificial heat 
in damp summer weather, when the air is saturated with moisture, 
may be quite as necessary as in winter. The amount of moisture 
in the air is most easily ascertained by a device known as the 
" wet and dry bulb thermometers." This consists of two thermo- 
meters mounted on a board, one of which has the bulb covered 
with muslin, and kept moist by a lamp-wick attached to it and 
dipping in a vessel of water. The temperature of the wet bulb 
is lowered by the heat consumed in evaporation, and the- differ- 
ence of its temperature from that of the dry bulb is proportionate 
to the drying power of the air. This may be approximately 
calculated in grams per cubic meter by multiplying the difference 
by 0-64 for Centigrade or 0-35 for Fahrenheit degrees, and if 
deducted from the total capacity for moisture corresponding to 
the temperature of the wet bulb as given in table, p. 517, will give 
the actual moisture in grams contained in a cubic meter of air ; 
but for practical purposes, all that is necessary is to find by 
experience the temperature and difference between the wet and 

^ The reason that a fog feels so much colder than its thermometer- 
temperature is, that in warming it to body-temperature we have to 
expend the latent heat required to convert these liquid particles into 
vapour, and in a frost-fog also to melt the ice. 



EVAPORATION, HEATING, AND DRYING 519 

dry bulbs which gives the best result for the drying required, 
and to maintain it as nearly as possible by regulation of the 
heating and ventilation. Cheap forms of the instrument are 
made for use in cottou-mills, where it is necessary to maintain 
a certain degree of moisture ; or it may be improvised from two 
chemical thermometers which agree well together. Distilled 
(rain or steam) water should be used to moisten the bulb, or it 
will quickly become coated with lime salts, and it should be 
placed in a moderate draught, or its indications will not be 
accurate. 

It is of course obvious that not only the wet thermometer 
but the wet hides or skins are cooled by evaporation, and they, 
in their turn, cool the air with which they are in contact, which 
not only becomes moistened, but is lessened in its capacity for 
moisture by cooling, and thus rapidly reaches a condition when 
it can absorb no more moisture. It is thus necessary to maintain 
its temperature by artificial heat, or to replace it constantly by 
fresh air from the outside, and which of these expedients is most 
economical will depend on the temperature of the air outside as 
compared with that which it is required to maintain. If the 
outside air is sufficiently warm, and not saturated with moisture, 
it is generally best to use it in large quantities without artificial 
heat, wind usually supplying the necessary motive power for its 
circulation.^ Wet goods from the pits may thus be dried to a 
" sammed " condition by any air which is not saturated and is 
above freezing point, though the drying will often be slow. 
For drjnng " off," artificial heat is usually necessary, since 
the attraction of the fibre for the last traces of moisture is very 
considerable, and to remove it the drying power of the air must 
be considerably higher than that required for the evaporation 
of free water.^ In dr5Aing stuffed leather a temperature must 
generally be maintained sufficient to keep the fats employed in 
partial fusion, and so permit their absorption by the leather, 
while at the same time the dr5dng must be gradual, or the water 

^ Compare also p. 553 on construction of tanneries. 

^ Commercially-dry leather generally, if unstuffed, contains about 15 
per cent, of residual moisture, which varies in amount with the weather, 
and can be more or less completely removed by drying at high tempera- 
tures. If leather has been over-dried, it only slowly regains its weight on 
exposure to cold air. Commercial disputes not unfrequently arise on the 
dryness of leather. In the opinion of the writer, a customer can only 
claim that the leather should be sufficiently dry not to lose weight when 
exposed to dry air at the ordinary temperature and degree of dryness of 
a warehouse or factory, and claims based on re-drying in hot drying rooms 
are distinctly fraudulent. 



520 PRINCIPLES OF LEATHER MANUFACTURE 

may be dried out before the fats have time to take its place. 
This is generally best attained by the use of artificial heat, and 
ventilation by circulating the air by a fan without its too frequent 
renewal, especially in cold weather. Frequently air which has 
been heated and used for drying off finished goods, and so 
partially saturated with moisture, may be used with advantage 
for wet goods, or for other purposes where a more gentle dr3nng 
is required. If the temperature is low outside, the amount of 
heat consumed in heating cold air to the temperature required 
may be very considerable. The weight of a cubic meter of air 
at 0° C. and atmospheric pressure is 1-293 kilos, and its specific 
heat at constant pressure is 0-2375 of that of water. Therefore to 
heat a cubic meter of air at ordinary pressure and temperature 
1° C. will require the same amount of heat as that used to heat 
0-307 kilo of water to the same extent, or in other words 0-307 of 
a k. -calorie. 1 If steam-heating is used, i kilo of good coal burnt 
under the boiler should heat about 1800 cubic meters 10° C, or 
I lb. should heat 52,000 cubic feet 10° F., assuming that the 
condensed water is not cooled below 100° C. These seem large 
volumes, but if we reflect that a 48-inch Blackman fan may 
move 30,000 cubic feet per minute, we shall realise that the 
cost of coal in heating air is not inconsiderable. 

We must now consider the heat consumed by the actual 
evaporation of the water in the leather. The actual evaporation 
of water already raised to 100° C. consumes 536 k. -calories, but 
the evaporation of water which has not previously been heated 
so far consumes more heat, and we may take that required at 
ordinary temperatures as in round numbers 600 k. -calories per 
kilo, or 1080 B.T.U. per lb. Disregarding small fractions, this 
is equivalent to the cooling to the same temperature of an 
equal weight of steam in the heating pipes, and this, as we have 
seen, demands about yV of its weight of coal for its production 
from water already heated to 100° C. 

The cooling takes place, in the first instance, in the leather, 
the temperature of which is reduced like that of the wet-bulb 
thermometer, and this in its turn cools the air in contact with it. 
Thus in air-drying without artificial heat the whole heat must 
be supplied by the air, and the loss reduces its capacity for 

1 For those who prefer Enghsh measures, a room of 10 feet cube or 
1000 cubic feet or 28-12 cubic meters contains 80-43 of ^ir at freezing 
point and mean barometer pressure, and requires 19 B.T.U. to raise the 
temperature 1° F., or 8-6 calories t6 raise it 1° C. As the temperature 
rises the weight of air becomes less, and takes somewhat less heat to 
raise it 1°. At 65° F. only about 18 B.T.U. is required. 



EVAPORATION, HEATING, AND DRYING 521 

moisture, greatly increasing the volume required. This is not of 
much consequence in open-air drying, since even a light wind 
will supply air in enormous volume. A moderate breeze of 
ten miles an hour moves about 15 feet or 4I meters per second. 
When, however, the air must be moved by fans, the power 
required becomes important. The evaporation of i kilo of 
water at summer temperature will cool about 2000 cubic meters, 
and that of i lb. 32,000 cubic feet of air 1° C. 

In calculating the ventilating and heating power required in 
fitting up dr3nng rooms it is usually necessary to ascertain that 
required under the most unfavourable circumstances, and then 
add a liberal margin to cover errors and accidents. As the 
calculations are, in consequence of the many varying conditions, 
somewhat complex, it may be convenient to give as examples 
the quantities of air and heat required to evaporate i kilo 
(2-205 lb.) of water under different ordinary conditions, and these 
may serve as a basis of calculation of the drying power which 
must be provided for different tanneries. 

1. Indifferent Open-air Drying. — Air at 10° C. (50° F.), wet- 
bulb thermometer 7° C. (44-3° F.), indicating a total capacity for 
moisture of about 2 grm. per cubic meter ; air not to be cooled 
beyond 7-75° C. (46° F.), leaving a residual capacity for moisture 
of 0-5 grm. per cubic meter. Each cubic meter will therefore 
take up 1-5 grm. of moisture, and as i kilo contains 1000 grm., 
we have 

1000 

=666 cubic meters per kilo required to absorb moisture ; 

600 
and — z =888 cubic meters reduced 2-25° to furnish the 

2-25° X 0-3 

600 calories required for evaporation. Total air used 1554 cubic 
meters or 54,900 cubic feet.^ 

2. Drying with Heat. — Outside-air at 10° saturated with 
moisture heated to 20° C. (68° F.) acquires a capacity for 7-9 
grm. per cubic meter. If we assume that a drying capacity of 
2 grm. per meter is required to complete the drying, we have an 
effective capacity of 5-9 grm. 

= 170 cubic meters or 6000 cubic feet, and to heat this 

5-9 
10° C. will require 510 calories. Evaporation of i kilo will con- 
sume 600 calories. Total heat mo calories. ' 

3. Drying with Heat. — Outside-air at 10° as above heated 
to 25° C. giving an effective capacity for moisture of 13-5 to 
2-0 = 11-5 grm. per cubic meter. 

^ 1000 feet^=28-3i4 m.^, or i m. ^=35-310 feet^. 



522 PRINCIPLES OF LEATHER MANUFACTURE 

==87 cubic meters or 3070 cubic feet. To warm this 

15° requires 391 calories, and 600 calories added for evaporation 
gives a total of 991 calories. 

Comparing 2 and 3 we see that the higher temperature is 
more economical, where it can be allowed, than the lower, 
both in air and heat, though this is partly compensated by 
the greater loss of heat by cooling of the building, etc., which 
it entails. 

4. Air at 0° C. heated to 20° requires about 97 cubic meters 
or 3430 cubic feet of air, and a total of 1180 calories. 

5. Air at 0° C. and heated to 25° C. requires 63 cubic meters 
or 2230 cubic feet, and a total of 1075 calories. 

6. Air at —15° C (5° F.) requires 4-5 calories per cubic meter 
to raise it to 0° C, and acquires a capacity for drying of about 
2 grm. per meter. 

We will apply these figures to a drying room arranged wth 
a screw-fan with a central division, or two floors, so that the air 
can be either circulated or replaced with fresh air from the out- 
side at will (see fig. 112). Such a room with 100 feet of length 
clear of space required for fans, air passages, and heating pipes, 
and 20 feet by 8 feet in section, should hang about 800 medium 
butts, weighing say I2| kilos (27 lb.) each, and when wet from the 
yard containing the same weight of water. A 48-inch Blackman 
fan under these conditions would probably move say 20,000 
cubic feet (565 cubic meters) of air per minute at the cost of 
2 or 2| horse-power. This, in a room of the section named, would 
give an average velocity of 125 feet per minute or rather under 
i| miles an hour ; not' at all too much to keep the air freely 
circulating among closely hung leather. If we assume that these 
butts are to be dried in a week (practically 10,000 minutes) under 
the conditions of No. 2, the 10,000 kilos of water they contain 
will require 1,700,000 cubic meters of air, or about 170 cubic 
meters per minute, or about y%- of the air must be fresh every time 
it passes through the fan. One kilo of water requiring mo 
calories must be evaporated per minute. 

Under the conditions of No. 4 only 97 cubic meters of air 
per minute would be required, or about f might be circulated 
without change, but the total heat required would be about the 
same, 1180 calories. Under the conditions of Nos. 5 and 6 some 
1620 calories per minute would be employed. It is hardly neces- 
sary to provide for the full amount of heat required by No. 6, 
since in this country such conditions occur but seldom, and never 
for more than a few days at a time, and during such a period 



EVAPORATION, HEATING, AND DRYING 523 

much less heat would suffice to carry on the drjdng at a slower 
rate and keep out the frost. 

Beside the heat required for actual drying it is necessary to 
provide for that lost by the building during cold weather, and 
this is much more difficult to calculate. If, by arranging the 
outlet for moist air on the pressure side of the fan, the internal 
pressure of the building be kept a little lower than the outside 
there can be no loss by escape of hot air, any leakage being 
inwards, and suppljdng a part of the change of air which, we 
have seen, is necessary. In a brick building with glass windows 
the loss of heat is far less than in the old-fashioned wooden 
louvre-boarded structure, and where fan-dr5ang is in constant use 
the brick structure is much to be preferred. Frequent windows, 
with casements horizontally pivoted at the centre, will supply 
enough air for favourable conditions of air-drying, and when the 
weather is bad, resort is had to the fan. Most modern drying 
rooms in the Leeds district are built upon this plan. Where 
louvre-boarded structures must be used for fan-drying the sides 
should be made as tight as possible in winter by sheets of canvas 
or sail-cloth nailed on, for which purpose old sails can be bought 
in seaport towns at reasonable rates, a few louvre-boards only 
being kept open for the admission of air in suitable positions. 

Box, in his Practical Treatise on Heat^ puts the loss through 
walls in brick buildings for a difference of 30° F. (i6-6° C.) between 
inside and outside temperatures at the approximate amounts 
shown in the following table: — 

Loss OF Heat through Walls 

Thickness of K. -calories per 
Wall in Inches. Sq. Foot per 
Hour. 

4-5 1*76 Stone walls must be about one- 

9 1-44 half thicker, to afford equal 

warmth with brick ones. 
14 i'20 The loss from glass windows 

18 • I -06 amounts to 3 or 4 k. -calories 

per square foot per hour. 

If walls are built with an air-space, as is now common, the loss 
of heat would be lessened. 

If the building is of several stories, the loss to the roof in the 
intermediate ones need hardly be taken into account ; but if the 
ceiling is not tight, and open to the roof, the loss may be great, 
1 E. & F. N. Spon, Ltd., London. 



524 PRINCIPLES OF LEATHER MANUFACTURE 

but difficult to estimate. If we consider the drying room already 
described, the total area of the walls and ceiling is about 4000 
feet, and to maintain its temperature 30° F. above the atmosphere 
at 1-2 calories per square foot would require 4800 calories per 
hour or 80 calories per minute, a very small amount compared 
to that consumed in drying. 

The following table calculated from data given by Box will 
give some idea of the amount of steam or hot-water piping 
required for heating. The sizes given are for the internal 
diameter of the pipe, allowance being made for the increased 
heating surface of pipes of ordinary thickness. Small pipes are 
considerably more effective in proportion to their surface than 
large ones, and for high -pressure heating i| or 2-inch wrought- 
iron pipes are to be recommended as in many ways preferable 
to cast iron. The gilled or ribbed pipes now often used are also 
advantageous as giving a greatly increased heating surface. 

The temperature of the air to be heated is understood to be 
60° F. ; at lower temperatures the quantity of heat given off by 
the pipes would be greater, and at higher temperatures less, the 
amount being approximately proportional to the difference of 
temperature between the air and the hot pipes. It is also im- 

Heat given by Steam-pipes 



Steam 


Tempera- 


K. -calories per hour 


per 


Pressure, 


ture of 


foot 


run of Pipe. 




1. per sq. in. 


Pipe. 










°C. 


-^F. 


2 in. 


3 in. 


4 in. 


52 


149 


300 


102 


137 


169 


35 


138 


280 


92 


121 


148 


21 


127 


260 


81 


106 


130 


10 


116 


240 


68 


92 


113 


2-5 


104 


220 . 


59 


81 


97 




99 


210 


54 


72 


89 




93 


200 


49 


66 


81 




88 


190 


45 


60 


74 




82 


180 


40 


54 


67 




77 


170 


36 


49 


60 



portant to note that the table refers to steam-pipes in still air, 
and that if placed in a powerful draught (as immediately before 
or behind the fan) their heating effect may be at least doubled. 
This has not been considered in the following calculations. 

Applying these figures to the estimate of mo calories per 
minute required for drying in our building, and assuming 80 



EVAPORATION, HEATING, AND DRYING 525 

calories per minute for the loss of heat through the walls, we have 
a total of about 71,400 calories per hour, and to obtain this 
would require 736 feet of 4-inch pipe at 220° F. (heated by 
exhaust steam), or 700 feet of 2-inch pipe heated to 300° F. by- 
steam at 52 lb. pressure. 

If we adopt the estimate of 1620 calories of Nos. 5 and 6 we 
shall require 1050 and 1000 feet of the two pipes respectively, and 
this covers approximately the worst conditions. We must, how- 
ever, remember that these estimates are made for continuous 
drying during the twenty-four hours, and that if the fan and steam 
are only applied during a portion of this time the supply both 
of air and steam must be proportionately increased, or the time 
of drying correspondingly lengthened. 

It is very desirable, however, that the fan should be driven by 
a small separate engine, the steam for which will only form a 
small proportion of that required for heating, and of which the 
whole of the heat will be recovered, since even that utilised in 
driving the fan will again be converted into heat by the friction 
of the air, and will therefore cost nothing. This arrangement 
will enable the drying to proceed so long as the necessary steam 
is maintained, which in bad weather can easily be done by the 
night watchman. It may also be pointed out that, during a great 
part of the year, the goods can be dried to a " sammed " con- 
dition without heat or in the open air, or in the case of dressing 
leather a considerable part of the water can be removed by 
pressing or squeezing, effecting a further economy. 

It must be left to the reader to apply the same calculation 
to other sorts of leather than sole, but it may be pointed out 
that the essential point, as regards heating and ventilation, is 
the weight of water to be evaporated in a given time, and that 
the actual size and shape of the drying room is unimportant so 
long as adequate heating and circulation of the air between 
the leather is secured ; and these remarks also apply to the 
particular form of fan or other ventilation employed, and to 
the means of heating. As the quantity of heat consumed is very 
considerable, it is well to look out for sources of waste heat which 
can be employed, or for means by which the heat of the fuel can be 
more directly and completely utilised than it is in raising steam. 
Thus a large amount of heat can sometimes be obtained by pass- 
ing air through pipes or " economisers " fitted in a chimney-flue ; ^ 
or gilled stoves or " calorifers " may be used in a separate chamber 
to directly heat the air which is drawn in by the fan. 

1 These pipes should be provided with scrapers to remove soot, as in 
Green's economiser, or their efficiency will be much diminished. 



526 PRINCIPLES OF LEATHER MANUFACTURE 

Figs. Ill and 112, furnished by the James Keith and Blackman 
Co. Ltd., give a good idea of the construction of screw-fans, of 
which there are now many different patterns, and the general 
principle of arrangement of fan-drying rooms, the air in this case 
being circulated in opposite directions on two floors, and the 
amount of change being regulated by the shutters at A, etc. 
The grouping of pipes at the ends of the two floors which it shows 




Fig. III. — Blackman Fan. 

is in general a good arrangement, but the length between them 
should not be too great, or the drj^ing will be unequal in different 
parts of the room. Sometimes this is convenient ; thus if most 
of the heat be supplied to the air coming fresh from the inlet 
of the upper floor, the damper and colder air of the lower room 
can be continuously used for drpng wet goods from the yard, 
and the upper reserved for drying off the finished leather. A 
disadvantage of this plan is that open-air drying can seldom be 
utilised except in an elevated building ; and even when it is 
adopted, means should be provided for heating the lower room 
in cold weather. In place of two~ floors it is obvious that a 
single floor may be divided into two compartments by a longi- 
tudinal partition. Whatever pipes are grouped at the ends of 
the building, it is advisable to arrange sufficient to prevent 
frost, against the walls, or in the old-fashioned way on the floors 



EVAPORATION, HEATING, AND DRYING 527 
beneath the leather, but not too close to it, and protected by a 



—If 



h 



ai 



o c o o J 100 
00000000 
ooooooco 
oooooooo 




I 





a= 



J 



o 

be 

S 



,2 J 



wooden lattice on which the workmen can stand, which also 



528 PRINCIPLES OF LEATHER MANUFACTURE 

removes the risk of accident from wet leather falHng on the hot 
pipes. The latticed space should be open at the end facing the 
air current so as to receive a portion of the draught, which will 
become heated and ascend, its place being taken by damp and 
cold air from the leather to be re-warmed. Water-vapour in 
itself is lighter than air, but the contraction produced by the 
cooling of evaporation more than compensates this, and the 
damp air is therefore heavier than the dry. The arrangement 
of hot pipes near the ceiling of a drying room, which has been 
borrowed from some American tanneries, is wrong in principle, 
unless the air is forced in at the upper part of the room or the 
upper floor is latticed, and only acts in other cases when the air 
is thoroughly mixed and circulated by mechanical ventilators ; 
while pipes near the floor will continue to produce a certain 
amount of circulation of the air, even when the fan is not running. 
In protecting pipes by lattices care should be taken not to 
confine them too closely, or their heating effect will be seriously 
diminished. In fan-drying, leather is preferably hung edgeways 
to the current of air, so as to allow of its free and uniform passage 
between. In the case of sole leather the butts or bends are 
conveniently suspended by S-hooks of brass or iron wire to hooks 
or nails fixed in the joists. If gangways between the leather 
must be left in the direction of the draught, they should be closed 
at intervals in the length of the room by curtains or shutters, so 
as to deflect the air-current into the leather. 

Screw-fans like the Blackman can be used either to suck or to 
blow the air, though the former is preferable where it can be 
arranged, because it produces a more uniform current in the room. 
On the blowing side the air issues with considerable velocity in a 
sort of cone, but little coming through the centre of the fan, while 
that near the edges spreads rapidly from its centrifugal motioji. 
This is rather advantageous where the fan blows into an open 
room, but involves waste of power where it discharges into narrow 
and square airways. The ends of the vanes of the Blackman may 
be turned in at the rim of the fan to prevent this tangential dis- 
charge, but it is probable that where a fan is to blow into a room it 
would be more advantageous to put it on the inner side of the wall, 
and without curved ends to the vanes, so as to distribute the air as 
widely as possible. A somewhat similar result would be attained 
with a Blackman by placing it in a position the reverse of that 
for which it is intended, and running it also the reverse way, but 
its " efficiency " might possibly be lessened. 

Screw-fans are good for moving large volumes of air at com- 
paratively low velocities and against little or no resistance, but 



EVAPORATION, HEATING, AND DRYING 529 

they are quite unsuitable for forcing air against high resistance or 
through narrow channels, and for this purpose centrifugal fans like 
the Capel (fig. 113) are much more suitable, and mechanically 
more efficient. In any case there is much loss of power in forcing 




Fig. 113. — Capel Centrifugal Fan. 



air through narrow airways, and if a screw-fan must be employed 
for the purpose, the channel should be as large in section as the 
area of the fan, and all sharp angles in its course should be 
avoided. There is great loss of power where a current of air or 
water has to pass suddenly either from a wider to a narrower 
channel, or the reverse, and in both cases the resistance is 
diminished by making the enlargement or contraction gradual or 
" bell-mouthed." Thus a pipe conveying water at a given head 

34 



530 PRINCIPLES OF LEATHER MANUFACTURE 

into or out of a cistern will discharge a much larger quantity if 
the ends are bell-mouthed than if it terminates abruptly. For 
the same reasons, air suffers considerable resistance if it has to 
pass suddenly into or out of a larger space, such as a drying 
room, and unnecessary partitions and other abrupt changes of 
dimension in the current should be avoided. Curves should also 
take the place of angles as much as possible. 

Systems in which air is drawn or forced over systems of heat- 
ing pipes by a centrifugal fan, and then distributed through 
comparatively small airways among the leather which is to be 
dried, are in some cases convenient and advantageous. Among 
these may be mentioned the Sturtevant and the Seagrave- 
Bevington. There can be no valid patent on the general prin- 
ciple of heating by distributing air in this way, but only on the 
particular arrangement or appliances used in the special case. 
Centrifugal fans should be considerably larger in diameter than 
in axial length, those with long vanes of small radius being 
wasteful in power from the insufficient supply of air to the centre. 
There is also no reason why, in some cases, centrifugal fans should 
not be substituted for screw-fans in drying on the system which I 
first described, especially in cases where the air has to encounter 
considerable resistance, as, for instance, in traversing a filter to 
remove dust. One of the best filters for this purpose is a table of 
wire-gauze or fine netting covered to a depth of 3 or 4 inches with 
loose wool. Hair or cheaper fibrous materials may be sub- 
stituted for the wool, but are less efficient. The air must of 
course be sucked downwards through the gauze. When the wool 
becomes dirty it may be washed, if possible in a wool- or hair- 
washing machine, and again spread on the table in a damp 
condition, as it will quickly be dried by the current of air. Flannel 
is also useful where the wool-filter is impracticable, but requires 
frequent washing. 

Apart from wind, natural ventilation is seldom to be relied 
on for drying on any considerable scale. Heated air is, of 
course, lighter than cold, and this is the cause of chimney- 
draught, but to get a good circulation in this way a high shaft 
and high temperature is required. Nevertheless, in one of its 
best forms the method has been a good deal used in America 
in the so-cahed " turret-dryer," a building of seven or eight 
stories in height, constructed of wood with latticed floors, and 
heated by steam-piping at the bottom, where the air is admitted. 
The method is not likely to be much used in this country, as, 
apart from the questions of cost of building, fire risk, and trouble 
of raising and lowering the leather, a good draught will only be 



EVAPORATION, HEATING, AND DRYING 531 

obtained when the outer temperature is low in comparison to 
that inside, and in our milder and moister climate the conditions 
are not nearly so favourable as in the United States. As the 
air is rendered heavier by the cooling of evaporation to a larger 
extent than it is lightened by the water vapour there is a ten- 
dency in drying by upward ventilation for the warm air to form 
local upward currents, while the cold and damp air falls back, 
and from this irregularity of flow it is difficult to saturate the 
air equally. This may be avoided by downward ventilation, 
in which the warm air is admitted at the top of the drying room, 
and the cold and damp air allowed to escape at the bottom. 
This fact suggests that in using systems of dr5dng, such as the 
Sturtevant, it would be better to place the distributing pipes at 
the top rather than the bottom of the room, but in this case care 
would have to be taken that there were no openings left by 
which the air could escape at the top of the room without de- 
scending through the leather. If this be avoided, the warm 
air will float on the top of the colder and damper and press it 
uniformly down and out. I believe the merit of first having 
applied the principle of downward ventilation to leather-drpng 
is due to Edward Wilson of Exeter. It is necessary that the 
hot air should be forced in at the top, or the cold air sucked out 
from the bottom ; and the mere placing of hot pipes near the 
top of the room (p. 528) will not cause the required circulation. 
Wilson placed his heating pipes in a partitioned space at the 
side of the room, at the bottom of which cold air was admitted 
from the outside, which escaped into the room at the top. As 
the temperature of this side chamber was high and the air con- 
sequently light an upward current was produced in it, though 
probably somewhat inefficiently, as the height of the column of 
heated air could only be small. Assisted by a fan, and circulating 
a part of the air, the method should give good results, especially 
over two (latticed) floors. As the air could not be satisfactorily 
heated in its downward course the method would not be suited 
for more than about two floors, and the drying in the lower room 
would be cool and gentle. 

One or two points in the practical arrangement of steam- 
pipes may be mentioned, as they are often overlooked even by 
professional engineers. The steam must always be admitted at 
the highest point in the system, and there must be a steady 
descent, without hoUow places where condensed water can 
accumulate, to the steam-trap by which it is removed. In 
horizontal pipes, about i inch descent in 10 feet is sufficient. If 
water accumulates there is not merely serious danger in case of 



532 PRINCIPLES OF LEATHER MANUFACTURE 

frost, but during use a vacuum is frequently formed by the 
sudden condensation of the steam, into which the water is shot 
hke the hquid in a " water hammer," producing violent and 
noisy concussions, and in some cases even fracture of the pipes 
or loosening of their joints. If high-pressure steam is used a 
very small supply-pipe will feed a considerable system of heating 
pipes or radiators, but with exhaust steam great pains should 
be taken to have pipes of ample size to avoid back-pressure on 
the engines. In both cases it is often convenient to arrange the 
pipes, not as a continuous line, in which drainage is generally 
difficult, but in parallels hke the bars of a gridiron. With high- 
pressure steam there need be no fear, if the pipes are kept clear 
of air by allowing a little escape through small air-taps, of the 
steam faihng to find its way to ah parts of the pipe, as a vacuum 
is produced by condensation in proportion to the heat given off. 
With exhaust steam no steam-trap is desirable, but any steam 
not condensed should escape freely into the open air or a chimney 
(after separ.ating condensed water), and it is well to render the 
resistance in all the pipes of a gridiron approximately equal, 
which may be done by admitting steam at one corner and 
allowing it to escape at the opposite (diagonal) one. In the 
arrangement of steam-pipes in parallels the practicability of 
repair to one pipe or joint without interfering with the others 
must always be considered. If screwed wrought-iron pipes are 
used each parallel must be provided with a bolted flange, or 
" running socket," to permit of unscrewing. The difficulty of 
accurately adjusting the lengths of the several parallels must be 
considered, especially with flanged metal pipes, and also ^their 
motion by expansion when hot, which amounts to i or 2 parts 
per 1000 of length according to the temperatures of steam and 
air. Expansion joints with stuffing boxes are costly and trouble- 
some and apt to leak, and may in many cases be avoided by suit- 
able arrangement of the pipes. Thus instead of having the pipes 
rigidly fixed at both ends one end of the system may be left free 
to move, each pipe being separately returned to an exit pipe at 
the same end but lower in level than the supply ; or a single exit 
pipe may be thus returned, its expansion and contraction being 
practicaUy the same as that of the heating pipes. In moderate 
lengths of wrought-iron pipe sufficient rehef may often be obtained 
from the flexure of the pipe if in some part of its course it is carried 
at right angles to its general direction, which is often necessary 
for other reasons. If pipes are laid in long lengths, the loose end 
should be supported on rollers or short pieces of pipe, so as to 
avoid moving the supports or straining the pipe in expansion. 



EVAPORATION, HEATING, AND DRYING 533 

It is useless to attempt to regulate the temperature of low- 
pressure steam-pipes by turning down the steam, since, so long 
as the pipe is supplied with sufficient steam to fill it, its tempera- 
ture cannot be less than 100°, and even with high-pressure pipes 
the power of regulation by altering the steam-pressure is very 
limited. It is far better to arrange the pipes or radiators in 
groups, from some of which the steam can be turned off entirely 
when less heat is needed. It must not be forgotten that if these 
discharge into a common steam-trap it will be necessary to turn 
off their exits as well as their steam supply, or steam will come 
back into them from the other pipes, and probably prevent the 
escape of condensed water. In many cases it is more convenient 
to give the several sections independent exits or steam-traps. 

Many good steam-traps are now on the market, depending 
either on the expansion and contraction of metals, or on floats in 
a closed box, which open a valve as the water accumulates. 
Traps of the latter class with closed copper balls are to be avoided, 
as the ball is sure eventually to become filled with water. . Several 
traps have been devised in which an open vessel is used as a float, 
which is always kept empty by the discharge of the water through 
a pipe dipping into it. 

The condensed water from steam-pipes is rarely suitable for 
use in the tannery, from the dissolved and suspended iron oxide 
which it contains, from which it can only be freed by boiling 
and filtering or treatment with precipitants (p. 67). Its most 
appropriate use is generally return to the boiler. Systems were 
formerly in vogue by which it was allowed to run back to the 
boiler as it condensed, but these could only answer when the 
pressure in the pipes was equal to that in the boiler, which is 
rarely the case. It must generally be forced in by the feed- 
pump or injector. 

Hot. water has often been advocated in preference to steam 
for heating, but is more costly, as it requires a separate boiler 
and much larger pipe-surface for the same effect. Its only 
important advantage is that the pipes maintain their heat for 
some time even when the fire has gone down, while steam-pipes 
cool at once if steam is allowed to go down in the boiler. In 
any considerable tannery, however, this will seldom or never be 
the case, since if a good pressure of steam is up at night when 
the fires are banked up, the boiler will in itself contain a large 
reserve of heat, and, of course, working pressure will be required 
before the engines can start in the morning. Hot-water systems 
require careful planning to obtain reliable and uniform circulation. 

A heating system is in use in which water at high pressure 



534 PRINCIPLES OF LEATHER MANUFACTURE 

and temperature much above boiling point is circulated by a 
pump in thick wrought-iron pipes of very small internal diameter. 
In the only case which came under the writer's observation it 
was not very successful, and if the pipes become choked from any 
cause, they are extremely difficult to clear. In ordinary hot- 
water systems without forced circulation the higher the heating 
pipes are above the boiler the better will be the flow. 

Among other leather-drying appliances, the " automatic 
dryer " of Mr A. N. Marr of Leeds must be mentioned. This 
consists of a long chamber in which air, warmed in side chambers, 

"|vlARR"PATENT LEATHER SAMMING &rDRYING MACHINE. 



CROSS S> E. C -r t O r-i 



■ ■ H- 

On Vfng ^v//ay(fK. ^ ■> 




Fig, 114. 



and with arrangements by which the proportion of fresh air to 
that already used can be regulated, is circulated. From end to 
end of the drying chamber pass endless chains slowly moved by 
wormgear, which carry the hides suspended from laths, which 
are picked up automatically at the entrance and dropped at 
the exit, the heat and air circulation being so regulated that 
they come out dried at the farther end. The writerx has seen 
only one of these appliances in action, which was drying chrome- 
tanned stock very satisfactorily, but was rather too rapid for 
vegetable tannages, a defect which could easily be remedied 
by a longer chamber and less intensive drying. The apparatus 
is shown in figs. 114, 115, and 116. 

Another arrangement which fulfils the same purpose very 
satisfactorily, though less automatically, is to suspend the goods 
from laths which rest on overhead bearers passing from end to 
end of a long drying room through which warmed air is blown 
or sucked by a screw-fan. The goods are entered at the cool or 



EVAPORATION, HEATING, AND DRYING 535 



exit end (for the air) and gradually advanced by hand towards 

the source of heat, where 

they are sufficiently dry 

for the succeeding process. 

Sole butts can be brought 

in direct from the yard at 

the cool end and delivered 

at the other ready for 

rolling, and the labour of 

advancing by hand is very 

small. 

Another good method of 
artificial drying is on the 
principle of the Sturtevant 
drier, in which a heating 
chamber and a pressure 
(centrifugal) fan , are con- 
veniently combined in one 
machine. Of course this 
is by no means essential. 
The air may be heated in 
any convenient chamber 
by steam or any available 
source of waste heat, or 
even by a coke stove or 
" calorifere," and propelled 
by any suitable fan, but 
the essential difference 
from the systems which 
have been already des- 
cribed is that it is dis- 
tributed throughout the 
drying room by pipes, 
generally of sheet iron, 
and about 8 to 10 inches 
diameter, and discharged 
directly on the suspended 
leather. These pipes may 
be either on the floor or, 
if the ceiling is a tight 
one, just below it, and the 
latter is, where circum- 
stances permit, the better 
way, as the warm and dry air floats on the top of the cooler 




536 PRINCIPLES OF LEATHER MANUFACTURE 



and damper and carries it down regularly below it, while warm 




5«>^: 



air rising through wet leather is apt to get chilled and form 



EVAPORATION, HEATING, AND DRYING 537 

irregular downward currents. It is necessary that the air as 
it escapes from the pipes should be distributed, and this 
is sometimes done by small free-running fans over circular 
apertures in the tubes driven by the escaping air. Small 
perforations in the tubes will of course fulfil the same purpose, 
and a simple and adjustable device would seem to be a circular 
metal plate supported over (or under) a round aperture in 
the tube by a screwed central bolt on which its position could 
be fixed by two check-nuts. The possibility of adjustment is 
somewhat important, for, unless both the size of the tubes and 
the power of the fan is very ample, the pressure will fall off toward 
the farther end of the system, and the goods there will get in- 
sufficient air. Centrifugal fans are essential, as screw-fans will 
not give sufficient pressure to overcome the friction in the com- 
paratively small tubes, and, for reasons which have been already 
explained, it is necessary to provide means for regulating the 
proportion of fresh air to that which has been previously passed 
over the leather. One of the great advantages of the Sturtevant 
principle is its easy applicability to most existing buildings. 



CHAPTER XXX 

CONSTRUCTION AND MAINTENANCE OF TANNERIES 

As few architects have specially studied the construction of 
tanneries, and in most cases much of the arrangement depends 
on the knowledge of the tanner himself, a short chapter on the 
subject will not be out of place. 

In the selection of a site a clay or loamy soil is to be pre- 
ferred to a gravelly or sandy one, as lessening the liability to 
leakage and waste of liquor. Perhaps, however, the first con- 
sideration of all is the possibility of drainage and disposal of 
effluent waste liquors and washing waters, since it is now rarely 
possible to run these, without previous treatment, into a river or 
stream. Some information is given in Chapter XXXI. on the 
methods of partial purification which are available to the tanner, 
but these are always costly and troublesome, and the possibility 
of running direct into a sewerage system or a tidal river is of 
great advantage. Under the Public Health Act authorities are 
bound to receive manufacturing effluents into their sewers if the 
latter are of sufficient capacity and the effluents not such as 
either to damage the sewers or interfere with the processes of 
purification adopted by the authority. This Act is in many 
districts practically superseded by special legislation, but tanners' 
effluents are generally received into sewers if freed from solid 
matter. When mixed with other sewage they do not interfere 
with irrigation or bacterial treatment .^ In selecting a site within 
a sewered district, regard must be had to the possibility of 
causing a nuisance to the neighbourhood by foul smells. Really 
injurious smells should not be caused by a properly conducted 
tannery, but it is difficult to avoid odour, and a single badly 
disposed neighbour may cause infinite trouble and expense. 
Traditional rights are not to be depended on as a defence, as 
objectors may claim that a change of process, or of the 
arrangement of the tannery, is not covered, even if it is really an 
improvement. 

^ Perhaps in this respect arsenical lime-liquors are an exception, but 
the use of arsenic sulphide can now be dispensed with, substituting Ccilcium 
sulphydrate or a mixture of sulphide of ggdiurri ^nd ca,lcii^rn ghloride. 

538 



MAINTENANCE OF TANNERIES 539 

Another important consideration is the water supply, since 
for the large quantities used in a tannery, town water is generally 
very expensive. With regard to quality and impurities of water 
information may be found in Chapter VIII., but, as a general 
rule, the softer and purer the supply the better. It is also a 
great advantage when the source is at such a level that the 
water can flow into the tanyard, or at least into the beam-house, 
without pumping. Filtration too, when needed, is much facili- 
tated by a sufficient head of water. Well-waters usually have 
the advantage of low and constant temperature, but are often 
hard. 

Commercial facilities, such as nearness to markets and sources 
of supply of raw materials, and the availability of rail and water 
carriage, are of an importance at least equal to the points already 
considered, but hardly come within the scope of this work. 

The site chosen, the next question is the arrangement of the 
buildings. It is very doubtful, where ground is not inordinately 
expensive, whether it is wise to erect drying-sheds over the pits. 
In case of fire very serious damage is done to liquor and leather 
by the heat and burning timber. If the turret form of drier be 
decided on, strong foundations are required, and the ground- 
floor or basement is occupied with heating apparatus ; if fan- 
drying, no lofty buildings are needed, and the drying rooms are 
conveniently placed over the finishing and currying shops ; and, 
on the other hand, the tan-house may be easily and cheaply 
covered with slated or felted roofs, with nearly vertical sections 
of glass, to the north if possible, like a weaving-shed, through 
which sufficient light for convenient work and cleanliness is 
admitted. The direct rays of the sun should be avoided, but in 
the writer's opinion the balance of advantage is largely in favour 
of a liberal supply of light. Iron roofs are unsuitable, since the 
moisture condenses on and rusts them, and particles of oxide 
fall into the liquors and cause iron stains. 

Good ventilation along the ridge of the roof should be pro- 
vided wherever any steam or hot liquor is used, or the condensed 
moisture soon leads to decay. 

In arranging the general plan of the buildings much depends 
on local circumstances, but as far as possible they must be so 
arranged that the hides and leather work straight forward from 
one department to another with as little wheeling or carrying as 
possible ; that the buildings where power is used be near to the 
engine so as to avoid long transmissions, which are very wasteful 
and costly ; and that the different buildings be so isolated as to 
diminish the risk of the whole being destroyed in case of fire. 



540 PRINCIPLES OF LEATHER MANUFACTURE 

A chapter on the construction and maintenance of tanneries 
and leather works would be incomplete if it did not refer to the 
very important question of Fire Insurance.^ To an extent this 
may be regarded as a fixed charge against any business, very 
much in the same way as local and imperial rates. It is not, 
however, to be lost sight of that to some considerable extent the 
amount of insurance premium is regulated by the insured him- 
self. If a man conducts his business in unsuitable and badly 
constructed buildings, if attention is not paid to some of the 
elementary hazards connected with a fire outbreak, he must not 
blame the insurance companies for the demand of what he con- 
siders an excessive premium. If this faulty construction and 
imperfect equipment of buildings pertain to any considerable 
extent throughout a given trade where the process is more or 
less hazardous, it is futile to appeal to insurance companies, 
which, after all, are merely commercial and not charitable 
institutions, for a reduction in the rates. The only standard to 
guide the company is the loss-ratio, and given a high loss-ratio, 
there must be a corresponding premium paid. 

There is, however — thanks to modern science — a method 
available whereby the great bulk of fires may be checked in 
their inception, an apphance automatic in its operation and of 
proved efficiency. This appliance is known as the " sprinkler." 
A system of water-pipes is fixed under the ceihngs of the building 
to be protected, to which are attached sprinkling jets at suitable 
intervals, each of which is closed by a valve held in place by a 
joint of fusible metal, which gives way if the temperature rises 
beyond a given point. Water under sufficient pressure must 
of course be available. There are two or three recognised 
patterns approved by the Fire Offices Committee after patient 
investigation and practical test. These appliances have now 
been at work for many years in this country. One of the first 
trades to recognise their utility was that of the cotton-spinner. 
At one time serious fires in the cotton trade were of frequent 
occurrence. Now — owing to the efficient fire appliances — while 
fires may be as frequent in their inception as formerly, they are 
stopped at such a stage as to prevent any considerable loss. 
The consequence has been that the cotton-spinner, at one time 
the owner of a highly-rated risk, and one which few companies 
cared to insure, is now in the position of having his business 
eagerly sought for, and large discounts offered him off the charges 
he was once called upon to pay. 

1 With regard to fire insurance, I am much indebted to Mr A. W. Bain 
of Leeds for valuable information. 



MAINTENANCE OF TANNERIES 541 

More important still is the consideration to him that his 
business is not so liable to be interfered with or stopped as the 
result of fire. There are, it is estimated, at the present moment 
no less a proportion than 90 per cent, of the cotton-spinners 
whose premises are protected by sprinkler installations. 

Other hazardous risks such as corn -millers', woollen and worsted 
manufacturer's, saw-miller's, engineers', are adopting these ap- 
pliances freely, and it is a matter of surprise that so very few 
tanneries or currying shops have done the same. The conse- 
quence is that the loss-ratio in tannery risks still retains its 
unenviable notoriety ; the rates for fire insurance have risen 
considerably, and as a result the tanners' profits are correspond- 
ingly less. Considering the extent and importance of many of 
the tannery risks throughout Great Britain, one can only express 
surprise that these appliances have been so little adopted. There 
is of course risk of damage by water, which should be covered by 
the policy. 

The construction of a new tannery demands serious attention 
from an insurance standpoint. The boiler-house should be a 
detached building ; the grinding of bark and myrobalans should 
be conducted in buildings isolated from the general works ; in 
fact no better advice could be given to a tanner, either in the 
construction of new premises or the rearrangement and re- 
modelling of old, than to consult an experienced insurance man, 
whether official or broker, as to the best means of constructing 
and arranging to secure the most favourable terms. 

Another point which should be provided for, and which is often 
overlooked, is the feasibility of future extension without serious 
changes of arrangement. It may be taken as a probability of 
the future, even if it be not already a fact, that small tanneries 
cannot be made to pay, and that if a business succeeds, its 
extension will prove desirable ; and in an ill-planned yard this 
may involve either entire reconstruction of a very expensive and 
inconvenient sort, or the separation of new departments, so as to 
involve serious increase of carrying. A good arrangement is 
that of a long front building serving to connect the whole, behind 
which the various departments are erected at right angles, leaving 
room for extension backwards as required. 

As regards the carrying of goods, if the various soaks, limes, 
bates, and handlers are well arranged, it is hardly necessary to 
do more than draw the goods from one pit into the next through- 
out the whole of the process. To and from the layers the goods 
must generally be carried or wheeled. In the sheds, if it be a 
sole leather tannery, the butts should first come into turrets or 



542- PRINCIPLES OF LEATHER MANUFACTURE 

open sheds for the rough drying, then into a room sheltered 
from draughts to temper for striking. The striking machines or 
beams should be in an adjoining room, or immediately below ; 
then a small shed-space for drying before rolling ; next the roller 
room ; and then the warm stove for drying off. If two of the 
latter can be provided to be used alternately it will allow the 
goods to be aired off without taking down, and they may then be 
immediately handed or lowered into the warehouse without fear 
of over-drying, which is sometimes difficult to avoid where leather 
must be taken direct out of the hot drying room. The same 
principles are easily applied in yards for lighter leathers. 

To lessen loss of power in transmission the engine should be 
near the centre of the main range of buildings, with perhaps the 
grinding machinery on one side and the leather finishing on 
the other ; but this would be rather liable to increase the fire 
risk. A very good plan would be to have the engine-house 
in the centre as suggested, but separated from the buildings on 
each side by brick gables, and with the boiler-house behind it, 
and under a separate roof, say of corrugated iron. If it be 
impossible to have the engine near its work, it is in most cases 
better -to employ separate high-pressure engines, which may be 
within a glass partition, and wiU work all day with scarcely any 
attention. The loss of power in carrying steam for moderate 
distances through sufficiently large and well-clothed pipes is 
much smaller than that of long fines of shafting. The writer 
has known cases where fuUy half the indicated power of the 
engine was consumed in friction of the engine, shafting and 
belts. High -pressure engines are as a rule to be preferred to 
condensing for tannery use, since the waste steam can generally 
be employed for heating, and both the first cost and that of 
maintenance are smaller. Where much fuel is used, it is quite 
worth while to have the cylinders indicated occasionally, both 
running light and driving the machinery ; much information is 
gained in this way as to the power spent on the various machines, 
and very frequently large economy is effected by proper adjust- 
ment of the valves. To work economically an engine should 
be of ample power for all it has to do, and adjusted to its work, 
not by lowering the pressure of steam or by checking it at the 
throttle- valve, but by setting the slide-valves to cut off as early 
in the stroke as may be. As to how early this is possible an 
indicator-diagram wiU at once give information. If the whole 
of the waste steam can be used profitably for heating purposes 
economy in the working of the engine is of little consequence, 
but, otherwise, it is very injudicious, for the sake of a little saving 



MAINTENANCE OF TANNERIES 543 

in first cost, to put in an old or inferior engine, which has to be 
dearly paid for in waste of fuel. In the choice of an engine the 
advice of an expert engineer is desirable, since many engines 
which are mechanically well made are uneconomical through 
the faults of a rule-of-thumb design. In this respect the English 
engine-builder is frequently inferior to his better-trained con- 
tinental competitor. 

In place of using small steam-engines to distribute power 
electric driving deserves consideration, and in many cases power 
can be much more cheaply obtained from electric supply com- 
panies than it can be generated on the premises. For long drives 
the loss of power is much less than that of shafting, and by con- 
centrating the whole production of the power in one large and 
well-constructed engine the cost per horse-power can be much 
reduced. While large and well-constructed engines may develop 
I horse-power at a cost in coal of less than i|- lb. per hour, it is 
not uncommon to use 12 lb. for the same output. In tanneries, 
however, the power used bears a much less proportion to total 
expenses than it does in the textile and many other trades. The 
first cost of electric driving is somewhat high. Motors of the 
" armoured " or iron-cased type must be used in all positions 
where they are subject to wet or dust. It must be borne in mind 
that an electric motor will not start against a heavy load, as it 
only develops its full power at a high speed, and if it receive 
the full pressure of the current before this is attained, its coils 
will probably be burnt out, unless saved by the melting of its 
safety-fuse. A similar danger is incurred if the motor is brought 
up by overloading while the current is on. It is therefore often 
necessary to connect a motor with its work by a belt which is 
only brought on to the working pulley when its full speed is 
attained, but many motors are now made with a high initial 
" torque," and switches are always used by which the full power 
can be gradually applied. 

In some cases the use of gas- or oil-engines is convenient and 
economical ; for though gas from town supplies is an expensive 
fuel, the best gas-engines give a higher mechanical efficiency 
than steam-engines, and they work with very little attention. 

In arranging shafting, moderate speeds, say 100 to 150 revolu- 
tions per minute, should be chosen for main lines, and when 
higher speeds are necessary they should be got by light and well- 
balanced countershafts, with wrought iron or wooden pulleys 
{cp. p. 545). In calculating speeds it must be remembered that 
they vary inversely as the size of the pulleys. Thus a 3-f. 
pulley running at 100 revolutions will drive a 2-f. pulley at 150 



544 PRINCIPLES OF LEATHER MANUFACTURE 

revolutions, and a 12-inch one at 300. Of course the higher its 
speed the more power any given shaft will transmit, but increased 
friction and wear and tear soon limit this advantage. The 
velocity of a belt in feet per minute is obtained by multiplying 
the number of revolutions per minute by the girth of the pulley 
in feet or by its diameter multiplied by 3i, or, more accurately, 
3-1416. 

Pulleys should always be of ample breadth for the power 
they have to transmit ; and it is more economical, both in power 
and cost, to use broad single belting than the same strength in 
double. If the pulley will not take a belt broad enough for the 
work it has to do, a second belt may be made to run on the top 
of the first, as suggested by Mr J. TuUis, and will do its share 
of the work. Belts should be washed occasionally with soap 
and tepid water, and oiled with castor or neatsfoot oil, but if of 
sufficient breadth, should not require the use of rosin or adhesive 
materials to make them grip the pulley. Chrome-leather belts 
should be kept thoroughly oiled. They have a much greater 
adhesion than vegetable tannages, and this is increased by oiling. 
Good chrome belting is much stronger than bark-tanned, and 
is unaffected by damp or steam, but generally stretches some- 
what more. Makers of machines often err in constructing their 
driving pulleys too small both in breadth and diameter. Very 
thin or soft belts will not run satisfactorily on flanged pulleys, 
as the edges turn up, and the belt often becomes crooked. This 
trouble is often due to pulleys not being in proper alignment, as 
the belt then tends constantly to run against the rigger to one 
side. Pulleys should always be slightly higher in the centre than 
at the edges, as the belt tends to run on the highest diameter. 

The horse-power which a belt is capable of transmitting 
obviously varies extremely with circumstances, but may be 

Ct V 

approximately calculated by the formula — '- — -, where a is the 

66000 

area of contact of the belt with the smallest pulley, and v its 

velocity in feet per minute. Another rule is, that at a velocity 

of 1000 feet per minute each inch of breadth of belt should 

transmit 2^ horse-power on metal pulleys, or 5 on wooden ones, on 

which the adhesion is greater. Adhesion may also be increased 

by covering the pulleys with leather or indiarubber, but in the 

latter case oily belts cannot be used. Holes are sometimes drilled 

in broad pulleys to allow of rapid escape of air. Both rules 

assume that the belt is of ample strength. One horse-power 

would be transmitted by a belt running 1000 feet per minute 

with a pull of 33 lb. A good single belt should not break with 



Kilo per sq. 


Lb. per 


Stretch 


centimeter. 


sq. inch. 


per cent 


283 


4.030 


25-4 


298 


4,240 


21 


740 


10,500 


325 


234 


3,330 


23 


• 835 


11,900 


38-3 


921 


13,100 


31-4 



MAINTENANCE OF TANNERIES 545 

much less stress than 1000 lb. per inch of breadth, and should 
not be given more than one-tenth as much as a working stress. 

The following table gives the experimental breaking stresses 
and extensions of some leathers. It may be noted that i square 
inch sectional area is equal to a belt 4 inches wide by ^ inch 
thick, and that kilos per cm.^ xi^-22=lh. per inch. 

Breaking Stresses of Leather ^ 



Belting leather, layer system 
,, ,, Durio system 

Well-tanned chrome leather 
Over-tanned chrome leather 
Stuffed alumed leather 
Alumed " rawhide " . 

Good English-tanned belting leather breaks at from 4500 to 
5500 lb. per square inch sectional area. 

Over-tanned leathers are less tough (though they stretch less), 
whether of vegetable or mineral tannage, than those somewhat 
lightly tanned, and the tensile strength of leather varies con- 
siderably with the part of the hide from which it is taken, that 
from approximately over the kidneys being the strongest. Even 
thick and tough leather is easily torn if a cut or nick is once 
started, and all holes used in jointing belts should be carefully 
rounded. Glucose and the use of acid in bleaching both lessen 
the toughness of belts, and they may also be rendered tender by 
the heat evolved in slipping on a pulley. 

Countershafting and high-speed machinery, such as disinte- 
grators, striking machines of the Priestman type, etc., should run 
without material jar or vibration. If this occurs, it is generally 
a sign that the rotating part is not equally balanced. In this 
case the shaft or spindle must be taken out of its bearings and 
supported on exactly horizontal straight-edges, on which it will 
roll till the heaviest part is downwards, and weight must then 
be taken off or added till it will lie in any position. In this way 
the writer has had to add fully 2 lb. of iron nuts to balance the 
drum of a striking machine before equilibrium was secured and 
a most troublesome vibration prevented. The exact alignment 
of bearings is very important, and a shaft running in bearings 
out of line wiU ultimately break. Of course all machinery should 
be supported as solidly as possible ; and if circumstances permit, 

^ Gerber, 1900, p. 73. 

35 



546 PRINCIPLES OF LEATHER MANUFACTURE 

most machines are better on a ground floor. In placing bark 
mills, however, it is frequently convenient to fix them at a higher 
level, so that the ground material may be sent down shoots by 
its own weight to the required places. An alternative, and 
perhaps better, plan is to set the mill on the ground over a pit, 
and to raise the ground material with a bucket-elevator. This 
may be done successfully by letting the material fall directly from 
the mill into the buckets ; but otherwise it must be thrown in 
with a shovel, as buckets will not pick up ground bark, even from 
a hopper ; and in any case such elevators are apt to be trouble- 
some, the tanning material getting between the pulleys and the 
belt, and causing the latter to break or come off. In a grinding 
plant designed by the writer the unground material was filled 
on the basement floor into an iron barrow, which was wheeled 
into an iron sling working between upright guide-rails like a hoist. 
On pulling a brake line the barrow was raised to the top of the 
building, and its contents were tipped into a large hopper, after 
which the barrow righted itself, and descended for another load. 
In the bottom of the hopper was a sliding shover, which forced 
the material on to vibrating screens, by which it was guided either 
into a disintegrator or crusher-rolls at pleasure. Both these 
discharged through iron spouts into large hoppers on the outside 
of a brick gable, from which powdery niaterials like myrobalans 
and valonia could be run direct into barrows or trucks. It is 
very desirable that such hoppers should be separated from the 
main building by a fireproof partition. Fires may occur from hard 
substances getting into disintegrators along with the bark, etc., 
and if this occur with a dry and dusty tanning matterial, it is not 
unlikely that it may result in an explosion such as sometimes 
happens in flour mills, in which the fire is rapidly conveyed along 
spouts and into chambers filled with dusty air. Insurance com- 
panies generally charge an extra rate for disintegrators, and it 
is very desirable to keep the mill-house structurally apart from 
other buildings, either by actual separation or by the introduction 
of brick gables dividing the roofs. On the whole, however, mills 
of the coffee-mill type are probably quite as dangerous as disin- 
tegrators, since if they become partially choked, the heat caused 
by friction is very great. 

In America the fire risk from mills is often lessened or pre- 
vented by the introduction of a jet of steam into the chamber or 
spout by which the mill discharges, but this is only permissible 
if the tanning material is conveyed at once to the leaches or 
yard. 

The use of chain-conveyors for handling tanning material, 



MAINTENANCE OF TANNERIES 547 

both wet and dry, is practically universal in America, though 
comparatively rare in England. Various forms are used, the 
most common consisting of a chain of square links of malleable 
cast iron which hook into each other, so that a broken link can be 
immediately replaced (see p. 389). At intervals special links are 
inserted, which can be had of various patterns, for the attachment 
of scrapers or buckets. The endless chain runs in a trough of 
rectangular or V-shaped section, and is driven by a toothed wheel, 
over which it runs like a belt. In some cases the returning half 
of the chain can be utilised to bring back the spent tan on its 
way to the boiler-house. For dry materials, cotton or leather 
belts working in troughs with short wooden cross-laths attached 
may often be used satisfactorily in place of the chain. The laths 
should be slightly wider than the belt. 

For lubricating purposes mineral oils of high density are not 
more dangerous than animal or vegetable, but rather the reverse ; 
as, though they are possibly more inflammable, and make more 
smoke, their mixture with cotton-waste and other porous materials 
is not spontaneously combustible, as those of vegetable and 
animal oils occasionally are. The danger of spontaneous com- 
bustion is very considerable when heaps of leather shavings 
or cuttings containing fish-oils are allowed to accumulate in 
warm workshops, and especially near steam-pipes. Heavy 
mineral oils should always be used as cylinder-oils in high- 
pressure engines in preference to other oils or tallow, since they 
are not decomposed by steam, and do no harm if blown into the 
feed- water, but serve to loosen and prevent scale and deposit. 
Ordinary oils and tallow, on the other hand, when submitted to 
the action of high-pressure steam, are separated into glycerin 
and fatty acids (see p. 428), and the latter corrode the valve faces 
and seatings, and are liable with " temporary hard " waters to 
form a very dangerous porous deposit in the boilers, which often 
leads to overheating of the tubes. 

Next to the machinery the pits demand special consideration. 
The chapter on the subject in the late Mr Jackson Schultz's 
book on Leather Manufacture, though now rather out of date, is 
well worth attentive study as giving American practice on the 
subject. 

The old-fashioned method of sinking pits is to make them of 
wood, and carefully puddle them round with clay, which should 
be well worked up before use. It is of no use to throw it in in 
lumps and attempt to puddle it between the pits, which will not 
be made tight, but probably displaced by the pressure. Such 
pits, if made of good pine and kept in constant use, are very 



548 PRINCIPLES OF LEATHER MANUFACTURE 

durable, some of the original, pits at Lowlights Tannery, con- 
structed in 1765, having been in use till 1889. Loam mixed 
with water to the consistence of thin mortar may also be em- 
ployed, the pits being filled up with water, to keep them steady, 
at the same rate as the loam is run in. One of the best materials 
for pit sides are the large Yorkshire flagstones. Where these 
are not attainable, very durable pits may be made of brick, either 
built with Lias lime and pointed with Portland cement, or built 
entirely with the latter. Common lime cannot be used, as it 
spoils both liquors and leather ; and even cements with too large 



9 wall 




d^cdl 



Fig. 117. — Mr C. E. Parker's construction of Lime-pits. 



a percentage of lime are unsatisfactory. Brick and common 
mortar are, however, suitable for lime-pits, and for these Mr 
C. E. Parker's plan of constructing the bottom of cement, the 
ends and sloping hearth of brick, and the sides of 3-inch planks 
bolted together is also very satisfactory (fig. 117). Most of these 
methods are now, however, superseded by reinforced concrete. 

The writer has constructed wooden pits in two ways. In 
the one case, after making the excavation, beams were laid in a 
well-puddled bed of clay ; on these a floor of strong tongued 
and grooved deals was laid, and on this the pits were constructed 
of similar wood to the floor, and puddled round with clay. In 
the second case the pits were built like large boxes above ground, 
and when finished lowered on to a bed of clay prepared for them, 
and then puddled both around and between. It may have been 
due to defective workmanship in the first case, but those made 
on the last-named plan, which is that adopted from very early 



MAINTENANCE OF TANNERIES 549 

times, certainly proved the tightest and most satisfactory. 
Mr Schultz describes a plan as the Buffalo method, in which a 
floor is laid as just described, and grooves cut with a plane for 
the reception of the sides, which are formed of perpendicular 
planks, each end and side being finally tightened up by the 
insertion of a " wedge plank." Owing to the perpendicular 
position of the side planks such pits would be difficult to repair 
in the common case of decay at the top. 

If bricks be used, great care must be taken that the cement 
is not merely laid so as to fill the joints towards the two surfaces 
of the wall, as is the habit of modern bricklayers, but actually 
floated into all the joints so as to make the wall a solid mass, 
or leaks can hardly be avoided. Hard-pressed bricks are best, 
and should be tested as to whether they discolour liquor. 
Cement-pits are very good, and though not particularly cheap in 
material, which must be of the best, are readily made by in- 
telligent labourers under good supervision. The first step is 
to lay a level floor of good concrete, in which glazed pipes for 
emptying the pits may be embedded, care being also taken 
that all joints in these are thoroughly tight, since future repairs 
.are impossible. The next step is to make frames the exact 
length and breadth of the pits required, and perhaps 15 inches 
deep. These are arranged on the floor where the pits are to be, 
and the intervening spaces are filled with concrete of perhaps 
one of cement to three or four of crushed stone or brick. Rough 
stones and bricks may also be bedded in the concrete as the work 
goes on to help to fill up. After the first layer has set, the frames 
may be raised and a second added, and so on. The work is 
generaUy finished by floating over it, while still damp, a little 
pure cement to give a smooth surface. Before using, the cement 
should be tried on a small scale to be sure that it does not dis- 
colour leather or liquors, and the pits should always be seasoned 
with old or cheap liquor before actual use. 

If possible, both leaches and handler-pits should be provided 
with plugs and underground pipes communicating with a liquor- 
well some feet below their levels. Glazed fireclay is very 
suitable for both pipes and plug-holes, which should be in the 
pit corners. If fireclay blocks for plug-holes cannot be obtained, 
they may be cast in good cement, the wooden mould being soaked 
with hot paraffin wax to prevent adhesion. Means must be pro- 
vided for the ready clearing of the pipes when choked with 
tanning materials. A good plan is to let each line of pipes end 
in a liquor-well large enough for a man to go down. As it is 
almost impossible to make plugs fit without occasional leakage. 



550 PRINCIPLES OF LEATHER MANUFACTURE 



r-N/-^ 




Vvv 



\ 



Fig. 1 1 8. — Cleaning Rod Joint. 



it is not well to run pits with very different strengths of liquors 
to one well, but the layers, handlers, and different sets of leaches 
should each have their own so as to avoid mixture. A good 
means of clearing pipes consists in a series of iron rods 3 to 4 feet 
long, connected by hooks fitting into double eyes, as shown in 
fig. 118. It is obvious that in a narrow pipe or drain these 

cannot become disconnected. 
The ordinary canes of the 
chimney-sweep are also useful. 
Pipes may often be forced out 
by fitting a strong delivery-hose 
of a steam-pump into one of 
the plug-holes. 

It is, as Schultz points out, 
of questionable advantage to 
lay wooden troughs under the 
alleys for 'supplying liquor to 
each pit, since it is almost im- 
possible to preserve them from decay, but the same objection 
would not apply to glazed pipes jointed with pitch or cemented. 
Between concrete-pits the alleys would probably also be con- 
crete. A very fine tar-macadam, such as is often used for 
footpaths, seems also suitable for alleys, and is easier to take 
up and make good for repairs of pipes than cement. A good 
and cheap plan in practice is to let the liquor-pump, or a raised 
liquor-cistern, discharge into a large and quite horizontal trough 
raised 6 or 7 feet above the level of the yard, and provided with 
plug-holes at intervals, from which the liquor can be run into the 
various pits by short spouts or sail-cloth hose. In place of 
plugs in the raised trough a simple and convenient valve devised 
by the writer may be advantageously employed. A lead weight 
is made by casting in a hemispherical pressed steel basin of about 
5 inches diameter and 2 inches deep in the centre, a loop of strong 
brass wire with turned up lower ends being suspended in the 
middle so as to become fixed in the lead. To prevent adhesion 
the tin must be previously burned off and the basin well black- 
leaded. This weight forms the valve, which rests in use on a 
6-inch washer of good indiarubber with a 4-inch hole, which is 
held by a wood block or brass flange against the bottom of the 
trough, through which a 5-inch hole is cut. The valve is raised 
by a lever or cord, and is absolutely water-tight in use. It is 
shown in section in fig. 91 (see p. 397) . 

It is very advantageous in practice, instead of pumping direct 
into the pits, to have one or more tanks into which liquor can 



MAINTENANCE OF TANNERIES 551 

be delivered by the pump, and which are sufficiently raised to 
allow it to be run from them into the horizontal distributing 
troughs which have been mentioned. This is specially important 
with regard to liquors for leaches and suspenders which are 
worked on a circulating system, since they do not run very 
quickly, and much time is lost in pumping out pits if the speed 
of the pump has to be regulated by the rate at which the liquor 
will circulate. It also enables liquors to be run through sus- 
pender- and rocker-pits during the night or at meal-times while 
the machinery is standing ; and it is often useful on beginning 
work in the morning to have an empty tank into which the 
first liquor can be pumped. 

Pumps. — Direct-acting steam-pumps without fly-wheels are 
very unsatisfactory for tanyards, since they are usually un- 
certain in their action, difficult to run slowly, and apt to 
" hammer," and they are also costly in steam, which cannot 
be used expansively. Steam-pumps with fly-wheels, operating 
the steam-valve by an eccentric, are free from these defects, 
and though more costly at the outset, soon save the difference 
in lessened repairs and consumption of steam. Pumps with a 
capacity of 8000 gallons per hour are very suitable, and can be 
used with a 3-inch hose-pipe ; smaller sizes are decidedly more 
liable to choke with tanning material. Rubber mitre-valves 
work satisfactorily, and do not choke frequently, but are costly, 
and easily damaged by hot liquors. On the whole, brass clack- 
valves are the most satisfactory, but the hinge-pins, instead of 
fitting neatly in circular sockets, should be held in slots, allowing 
the back of the valve to rise half an inch, when it will clear itself 
of small hard myrobalan stones and such-like things, which, 
getting under a more tight-fitting hinge, would prevent the valve 
closing, and so stop the pump. Rubber-covered balls working 
in a cage are also good. Whatever valves are employed, means 
should be provided for easy access without unscrewing too many 
bolts. If the several valve-chambers of the pump are closed by 
a single cover with an indiarubber washer the spaces between 
them which make the joint should be faced with brass or gun- 
metal, as, if the least leakage takes place over an iron surface, the 
friction and solvent power of the liquors soon eat away the 
metal and render a good joint impossible. Where colour is 
of first importance it is well to have the whole pump of gun- 
metal, but in any case the working cylinder should be brass- 
lined, and the piston and rod and the valves and seatings should 
be of brass or gun-metal. Spring-rings are far better than 
pump-leather, and are unaffected by hot liquors ; chrome leather, 



552 PRINCIPLES OF LEATHER MANUFACTURE 

however, will stand a good deal of heat. Double-acting force- 
pumps have practically superseded the older single-acting 
double of triple pumps. Instead of direct driving with a steam 
cylinder it is sometimes advantageous to drive by belt; but at 
least one steam-pump should be provided, so that pumping can 
be done when the main engine is not running, and the speed of 
the pump can be regulated to the work, which is impossible in a 
belt-driven pump. Steam-pumps are sometimes very useful 
as fire-engines. 

Centrifugal pumps are very suitable for tannery work where 
the liquor is drawn from a well, but are not well adapted for use 
with suction-pipes. If the form with vertical spindle is adopted, 
which is sunk below the liquor in the well, the pump fills itself 
and needs no foot-valve, but unless the well is very large, or 
some convenient means is devised of withdrawing the pump, 
repair or cleaning is difficult. If the horizontal pattern is used, 
which is above the ground, repair, cleaning, and driving is much 
easier, but a foot-valve is necessary, which may itself give trouble, 
and some convenient means, such as a pipe from a raised tank, 
should be provided for filling the pump with liquor, as, unhke 
suction pumps, centrifugals will not start unless full, although 
they raise very large quantities when running, and from their 
steady flow will deliver much more through a given pipe than 
an ordinary reciprocating pump with the same power. In 
selecting the pump care should be taken that the pattern allows 
ready access, not only to the foot-valve, but to the body of the 
pump. Some of the more modern rotary pumps may prove 
suitable. 

It is seldom satisfactory to use windbores or strainers to 
prevent tanning material getting into a pump, as they speedily 
become choked ; and it will be found better, after taking such 
precautions as are possible, to have the pump and valve of ample 
size and suitable construction to pass what comes with the liquor. 
The writer has known a mop-head pumped and delivered through 
a 3-inch hose without stoppage by a Tangye fly-wheel steam- 
pump with brass clack-valves such as have been alluded to. 

Pulsometers have not, in the experience of the writer, proved 
satisfactory in tanneries, warming and diluting the liquor, con- 
suming much more steam than a pump of the same power, 
and becoming easily choked. For the same reasons, steam-jet 
water-raisers are not to be recommended except where raising 
is to be combined with heating, as in some leaching devices 
(p. 400). 

But little alteration has been made in the foregoing chapter 



MAINTENANCE OF TANNERIES 553 

since it was written in 1903, but it may be in place to conclude 
with a few words as to the tendencies recently observable. The 
use of reinforced concrete, not only in general construction but 
in the making of pits, has considerably increased, partly due to 
the difficulty of building in brick, since it is possible to do much 
of the concrete work with the semi-skilled labour generally 
available about a tannery, and with the aid of the joiners' and 
fitters' shops with which modern tanneries are generally provided. 
For the construction of pits it is probably really the most satis- 
factory material, and for lime- and water-pits requires no further 
finishing, but for liquor-pits should be coated with boiled oil, 
with pitch, or with some sort of varnish, though some cements 
Mdthstand the action of ordinary tanning liquors quite well 
without. For chrome liquors protection by some varnish is 
essential, and cement on the whole is probably less satisfactory 
than wood or, what is still better, slate. 

In roofing for tan-houses the weaving shed construction with 
lights to the north has still much to recommend it, but for lime- 
houses the light wooden latticed girder and curved roof has very 
much taken its place, as it has the advantage of giving very large 
spans without the obstruction of pillars. One of the finest 
lime-houses which the writer has seen is that of Messrs Walker 
at the Rosehill Tanneries at Bolton, not only on account of its 
size, but from the admirable arrangement of the machines, 
first the lime-pits with suspension and agitation with compressed 
air, then a row of unhairing machines with pits behind them to 
receive the hair, next a row of fleshing machines with pneumatic 
rolls, and finally a wide space for sorting and rounding, so that 
the hides proceed direct from the limes to the rounding table. 
For flooring for most purposes about a tannery good concrete 
is quite satisfactory, but it is a good deal attacked by acid chrome 
liquors, and genuine asphalt or the fine tarmac often used for 
footpaths would probably be more durable. 

For leather drying the use of wooden louvre-boarded build- 
ings has become almost obsolete, and brick structures with 
pivoted windows have largely taken their place, while dr5nng 
with artificial heat and fan ventilation is almost universal. 
Details of the methods most in use will be found in Chapter 
XXVIII. The use of fire-proof reinforced concrete floors for 
upper stories is increasing. Compressed air is coming much 
into use, both for agitating and for moving liquors. Mechanically 
it is not very economical, as much heat is lost at the compressor, 
which must be water-jacketed, and the compressed air cools the 
liquors in expanding, which is sometimes advantageous and 



554 PRINCIPLES OF LEATHER MANUFACTURE 

sometimes the reverse, and it costs as much mechanical power to 
force air to the bottom of a hquor as to raise the same volume of 
water the same height, but once the somewhat expensive com- 
pressor is installed, its convenience is so great that this dis- 
advantage is probably fully compensated. For stirring of 
suspension limes the air is simply allowed to escape from per- 
forated tubes at the bottom of the pit, of which the bottom should 
be so curved as to promote a good circulation of the lime and 
liquor, and the only trouble which sometimes occurs is the ten- 
dency of the lime sludge, which must settle towards the air- 
pipes, to choke them when the air is not flowing. For this reason 
it is probably best to have the openings directed downwards. For 
the stirring of tanning liquors the oxidation has been pointed 
out as a disadvantage, but unless the liquors are alkaline, the 
loss is not likely to be important. With chrome liquors no 
oxidation takes place, but if used for hypo liquors in the two- 
bath process there may be loss of sulphurous acid. For stirring 
loose skins, compressed air is not so satisfactory as the paddle. 
The moving of liquors by compressed air is done by the " air- 
lift," the liquor being allowed to flow down into a well some little 
depth below the bottom of the pit, and the air being made to 
bubble up through an open-ended pipe, when it will carry the 
liquor up with it to some height above the surface of that in the 
pit. This has the great advantage over pumps that there are 
no valves and no wearing parts. The method has long been 
used in the United States for raising water from deep wells where 
the installation of a pump would be difficult. 



CHAPTER XXXI 

WASTE PRODUCTS AND THEIR DISPOSAL 

The products which are of no direct value to the tanner and 
currier in the manufacture of leather, and which are nevertheless 
obtained in fairly large quantities, are of very varying characters. 
In the present chapter the most important of them will be 
described and some of their uses mentioned. 

Hair is removed from the skin of the animal in the process 
of depilation (p. i66) in the form of a wet sodden mass, con- 
taining a considerable amount of lirne when the skin has been 
through the lime-pits. 

As white hair is the more valuable, it is desirable in the un- 
hairing to keep it separate from the coloured, but this cannot 
well be done in unhairing with machines. It is washed first in 
plain water to get rid of as much of the lime as possible, and then 
in . water containing a little acid. Hydrochloric acid is often 
used for this purpose, but sulphurous acid (p. 24) is preferable, 
as it has a slight bleaching action on the hair. The acid 
neutralises and renders soluble the lime which still remains in 
the air, so that it can be easily removed by washing with water. 
In many tanneries hair-washing machines are used. The 
washed hair is dried by laying it out on frames ; or preferably, 
the greater part of the water is first removed by a centrifugal 
drier or by pressing, and the drying is completed in a drying 
room, the temperature of which may be pretty high if it is pro- 
vided with a fan or some other appliance for mechanical ventila- 
tion. Tables of wire-gauze on which the hair is spread, and 
through which the warm air of the room is drawn by a centrifugal 
fan, are very effective. 

Coloured hair is sometimes washed and treated like the white 
hair, but is usually sold direct to plasterers, in which case there 
is no necessity to remove all the lime and other impurities which 
the hair contains. A considerable amount of hair is also sold to 
iron founders, who use it in preparing cores and in loam-casting. 
The loose lime may be effectively beaten from dried hair by 
passing it through a disintegrator with one of the grates re- 
moved. The use of moderate quantities of sulphide in the limes, 

555 



556 PRINCIPLES OF LEATHER MANUFACTURE 

though they weaken the hair, do not seem to affect its commercial 
value. 

Fleshings and Glue-stuff. — The various scraps of fat and 
flesh, containing some actual hide substance, are usually worked 
up for glue, though if they cannot be sold for a fair price it will 
pay to boil them in order to recover the fat they contain. Before 
boiling, the fleshings are treated with sulphurous, sulphuric, or 
hydrochloric acid, sufficient to neutralise the lime present. The 
boiling should be carried on very gently, so as to allow the fat 
to rise without emulsifying with the gelatinous matter. Open 
steam may be used, but in this case the size formed will have . 
little value ; on the other hand, if the fleshings are carefully 
delimed with sulphurous acid and a wooden vat with a copper 
steam coil be employed, really good size may be obtained, and 
the slight trace of bisulphite which it may contain will prevent 
its putrefaction. Except under special conditions it will not pay 
to make glue on a small scale in England, as its value depends 
much on its appearance, and the necessary plant is somewhat 
expensive. In some places, however, size can be sold to advan- 
tage. Fig. 119 shows a glue-boiling plant. 

Material for glue must be in a neutral condition. In dried 
glue-stuff this is secured by the carbonation of the lime in drjdng, 
but wet fleshings or glue pieces must be freed from lime by 
treatment with acid, preferably sulphurous, and then from excess 
of acid by thorough washing, which may be shortened by the 
use of a little soda in the last wash water, so that the material 
is neutral to methyl orange or Congo red, but must not redden 
phenolphthalein. In place of using sulphurous acid goods may 
be carbonated by blowing carbonic acid (the fumes of a coke 
stove, or even washed furnace-gases) through them in water till 
they no longer redden phenolphthalein, and this method is 
particularly advantageous with matter containing rancid fats 
with oxidised acids, which from their ready emulsification are 
particularly apt to render the glue turbid. The skimmed fat 
is also improved, but the " scutch " or undissolved residue must 
be treated with acid to recover the fatty acids present as lime- 
soaps. The writer has in this way made a bright glue from seal- 
fleshings, which usually give one turbid and nearly black. 

Skin-glues are generally boiled in open vats such as are shown 
in the illustration. These may be of iron or copper or of wood, 
like the leaches used in tan extraction (p. 395), and, like them, 
they are furnished with a copper heater or boiling coil, and with 
a perforated " false bottom," usually of iron or copper, to support 
the material. The steam for heating is supplied by a vertical 



WASTE PRODUCTS AND THEIR DISPOSAL 557 

pipe in the centre, which is surrounded by a wooden casing or 
" eye " passing through the false bottom, so that when the hquor 
boils it rises through the casing and flows over the top of the vat, 
passing down through the glue-stuff. If dry material is to be 
boiled the vat is usually fitted with a " curb," in which the 
" spetches " can be piled, and sink down gradually as they soften. 
For material in which the lime has been neutralised and rendered 




Fig. iig.^Glue Boiling. 



insoluble by carbonation iron vats are quite suitable, but if acid 
deliming, even with sulphurous acid, is practised, wood is better, 
because the iron (and even copper) is slightly attacked and 
darkens the glue. After boiling for six or eight hours as much 
of the fat as possible is skimmed off, and the liquor is run into a 
tank, where it is allowed to cool somewhat, and a further portion 
of fat separates. The material is boiled a second time with a 
further quantity of water, and for common glues this size is 
usually used to fill the vat for boiling a^second lot of glue material. 
A preferable method is to evaporate at once for a somewhat 
inferior glue, or to add it to the strong size for evaporation (see 
below). Some alum or alumina sulphate is frequently added to 
the size at this stage to harden the jelly and raise its melting point. 



558 PRINCIPLES OF LEATHER MANUFACTURE 

After separation of the fat by skimming, the clear size is run 
off from the residual matter into wooden or galvanised iron cool- 
ing troughs about 5 feet long by 9 inches deep and 15 inches wide, 
in which it is allowed to set (fig. no, p. 517). Great care is re- 
quired that both size and coolers are quite sweet and free from 
putrefaction, the coolers being frequently washed with sul- 
phurous acid solution or fresh milk of lime. The jelly is cut out 
of the coolers in blocks, and sliced into cakes of appropriate thick- 
ness by means of a series of frames like slate-frames which fit 
over the block of glue, and between which a wire or thin blade 
stretched on a saw-frame is inserted to cut the glue into sheets, 
or nov\^ more commonly by a machine with a series of parallel 
blades against which the glue-block is pushed. The sheets are 
afterwards separated by girls and laid to dry on nets, on which 
they are frequently turned. When dry the cakes may be 
washed with warm water to remove any adhering dirt, which in 
smoky districts quite spoils their appearance, but this causes 
some loss of weight, and in many cases it pays better to dry in a 
stove until quite hard, then grind in a disintegrator and sell as 
" size-powder," in which appearance counts for little if the colour 
and strength of the size are good. 

Artificial heat cannot be used in drying the soft glue on the 
nets, and in hot, and especially in thundery, weather it some- 
times melts and runs through the nets, not only wasting the 
material, but making a mess difficult to clean up ; and bacterial 
troubles often occur, probably increasing the tendency to melt, 
and causing bubbles in the interior of the cakes. For this and 
other reasons the process just described has been largely super- 
seded, and the settled or filtered size is run direct to an evaporator 
of the Yaryan or " Climbing Film " type (p. 408), and concen- 
trated till it will set to a firm cake when run on glass plates 
previously waxed or rubbed with ox-gall to prevent its adhesion. 
Vacuum is not strictly necessary, as the very brief heating to 
boiling temperature does not noticeably injure the glue. The 
cakes when set are stripped off the plates and dried on nets, which 
are often in frames on wheels, which are passed through a 
" tunnel " through which warm air is circulated, entering at the 
farther end, when the partially dried glue can stand a higher 
temperature. 

Fat.- — The fat, whether obtained in the manufacture of glue 
or by boiling the fleshings and shavings for its recovery alone, is 
skimmed from the surface of the heated liquor, and should 
afterwards be freed from gelatinous matter by washing it with 
hot water in a tub and running off the upper layer after allowing 



WASTE PRODUCTS AND THEIR DISPOSAL 559 

the water to settle out. The fat thus obtained is a hght-coloured 
grease of buttery consistence. 

There are various other sources of waste fats which may be 
considered here. If glue is made from dried glue-stuff without 
previous treatment with acid, the fat skimmed off the pans, 
though dark in colour, will be neutral or alkaline, and a con- 
siderable additional quantity of fat and free fatty acids may be 
obtained by reboiling the " scutch " or reluse with open steam in 
lead pans with the addition of water and enough sulphuric acid 
to render the contents of the pan distinctly acid. The scutch may 
also be pressed in steam-heated presses, such as are used to 
extract the " magma " precipitated from wool-washings. This 
grease will be dark and of unpleasant smell from volatile fatty 
acids, but its odour may be to a considerable extent improved by 
blowing air and steam through it and washing with water, or by 
heating to a temperature somewhat above the boiling point of 
water for a considerable time. The same sort of treatment may 
be applied to the fat pressed out of sheep-skins, and to that 
obtained by boiling currier's shavings with water and a little acid. 

Recovered fats may be separated into a tolerably firm grease 
suitable for use instead of tallow in currying, and an oil, not 
unlike neatsfoot oil, by melting, allowing to cool slowly to a 
soupy consistency to promote the crystallisation of the harder 
fats, and forcing the mixture through flannel cloths in a filter 
press. The temperature at which the filtration should take place 
is generally 20° to 25° C. The oil is, of course, " tender," or liable 
to solidify in cold weather, and the more so the higher the 
temperature at which filtration takes place, and this might be 
lessened by a second filtration at a much lower temperature. 
The tallow is obtained in cakes. If from fresh fleshings it will 
be white and with little odour, but that from dried glue-stuff is 
usually brown and of unpleasant smell, while recovered grease 
from curriers' shavings or " moisings " is always dark in colour. 

If the fleshings are to be sold wet, they should be preserved 
in a sweet lime liquor ; if to be dried, they are washed care- 
fully in a fresh lime, spread on frames, and frequently turned 
over so that they may dry evenly and rapidly. Heat, if employed 
at all, is in most cases only used at the end of the drying operation, 
but some tanners dry from the first in a room the temperature 
of which is a few degrees higher than the normal, and which is 
provided with good ventilation. For the purposes of the glue 
manufacturer the roundings and larger pieces are more valuable 
than the fleshings, and should be treated with correspondingly 
greater care by the beamsman and his assistants. 



56o PRINCIPLES OF LEATHER MANUFACTURE 

Bate-shavings are very valuable as sizing materials. They 
should be well washed in water, or with a very dilute solution of 
sulphurous acid, and are then laid out in thin layers to dry. They 
may also be partially dried by pressing between latticed boards 
in a screw or hydraulic press, and are then best finished as 
cakes. On the manufacture of sulphurous acid compare p. 24. 

Horns are usually kept until the " slough," " pith," or internal 
bone can be knocked out, having become loosened through 
drjnng and putrefaction. If kept dry, practically no longer 
time is required, and the smell and other annoyances incidental 
to storing in a damp place are avoided. The sloughs may be 
removed at once by steaming, but the horns are somewhat 
damaged by this treatment. The sloughs are principally ground 
for " bone-meal," but some are boiled for glue, either without 
preparation or after decalcifying with dilute hydrochloric 
acid. 

The actual horn itself, which is ' quite incapable of making 
glue, is used chiefly in the manufacture of combs, buttons, and 
similar articles. The value of horns is to a considerable extent 
dependent on their size, small horns being unprofitable to work 
up for the articles above mentioned. 

Spent Tan. — The tan as it is obtained from the leaches after 
extraction has, naturally, no value for the tanner except as a 
fuel. Spent tan cannot be profitably sold as manure, as its 
worth in this respect is extremely small. In those places where 
white lead is still made by the Dutch process, oak-bark is used 
to cover up the earthen pots, and commands a good price. It 
is, however, essential that oak-bark only should be used, as 
many other tanning materials give off products which injure the 
colour of the white lead. The quantities of tan used for hot- 
beds, and for deadening the noise of traffic in the streets, are so 
small, that they are of no practical account in the disposal of 
this product. Spent tan is not nearly as good as wood for the 
manufacture of paper, though spent mimosa bark has been used 
for brown paper with some success. An attempt to distil it, 
and thereby obtain pyroligneous acid and wood-spirit, did not 
result in any commercial success. On the Continent fine-ground 
tan is usually pressed into briquettes for use as domestic fuel, 
but it would be hard to obtain a market for these in England. 

On the whole, in spite of its low heating value, spent tan is 
best utilised as a fuel. For this purpose specially constructed 
furnaces are necessary on account of the dampness of the tan 
and its low calorific value, which varies, however, with the 
particular materials : thus while oak-bark and valonia are only 



WASTE PRODUCTS AND THEIR DISPOSAL 561 

poor fuels, hemlock and myrobalans are much better on account 
of the resins and ligmne they contain. 

The first successful furnaces for raising steam with wet tan 
were introduced in the United States, and consisted of a large 
arched combustion chamber with abundant grate-area, and with 
four or six feed-holes in the fire-brick top which formed a floor 
on which the spent tan was laid, and where to some extent it 
was dried by the waste heat. The flames and furnace gases 
were conducted under the boilers, the flue being very large and 
deep so as to collect the light ash which was drawn in great 
quantities from the furnace, and the gases were then returned 



Groiaixl -Lwve 




FumcLce' 

AshyPiAy 
Fig. 120. — Huxham and Brown's Furnace. 



through the tubes of the boiler, afterwards passing down the 
sides and going to the chimney. The wet fuel, partiaUy dried 
on the firing floor, was fed in through the firing holes alternately, 
so that only a part of the grate-space was covered at once with 
wet fuel, which was speedily ignited by the heat from other parts 
of the furnace, and especially from the vaulted arch.^ The large 
grate-area was a necessity not only on this account, but because 
of the Ught weight of the fuel and its low calorific power, which 
involved the need of burning a large volume. Fig. 120 represents 
a furnace of similar principle constructed by Messrs Huxham 
and Browns. Furnaces of this type are, the author believes, 
still largely in use in the United States, but in Germany " step- 
grates " sloping from the furnace-doors towards the back are 
now preferred. In these the combustible material rests upon 
the flat surfaces of the grate, while the air enters by the spaces 
between the steps without the fuel being able to fall through. 

1 Detailed drawings and particulars are given in Jackson Schultz's 
Leather Manufacture in the United States, New York, 1876. 

36 



562 PRINCIPLES OF LEATHER MANUFACTURE 

Fig. 121 represents the furnace on this principle constructed by the 
Moenus Co. of Frankfort. 

The essential conditions which are to be observed in the 
proper burning of the tan are a sufficiently large grate-area, a 
correct and sufficient supply of air, and a combustion-chamber 
of very high temperature. It is consequently not possible to 
burn tan very successfully in an ordinary Lancashire or Cornish 



-^. 





Fig. 121. — Moenus Step-grate Furnace. 



boiler, since not only the grate-space is too limited, but the 
water of the boiler prevents the upper part of the furnace from 
attaining a high temperature, and it is therefore difficult to get 
the damp tan rapidly into vigorous combustion. The difficulty 
may to some extent be overcome by mixing the tan with a 
proportion of coal, and by closing the ash-pit and employing a 
forced draught, unless the chimney is a very powerful one. In 
this way large quantities of tan may be burnt, but without 
effecting any great saving of coal. The heating power of the tan 
is improved by the partial removal of its water by pressing, and 
this is almost essential where a special furnace is not employed. 

The answer to the question as to whether tan should be used 
as fuel in the wet state in which it is obtained from the leaches, 



WASTE PRODUCTS AND THEIR DISPOSAL 563 

or whether it should be previously pressed, depends upon the 
nature and quantity of the tan. Where abundant quantities of 
a fairly good material such as hemlock bark are to be disposed 
of, the cost of pressing is an unnecessary expenditure ; but if it 
is desirable to obtain the highest value from the tan, or if the 
furnaces are not well constructed for burning very wet fuels, it 
will be profitable to press the tan. Hydraulic presses have been 
used for this purpose, but those now commonly employed con- 
sist of powerful rollers arranged in the same way as those of the 
myrobalans-crusher (p. 386). The pressure is given by levers 




Tan Press. 



loaded with weights or fitted with powerful springs. The liquid 
which runs from these presses is of little value, as it contains 
such large quantities of finely divided material that it is almost 
impossible to filter it, and if run upon the leaches it chokes them 
and prevents their proper circulation. Much of the cost of 
pressing is caused by the labour of feeding it to the press, and 
this may be greatly reduced by the use of mechanical con- 
veyors (p. 390) from the leaches. A tan press is shown in 
fig. 122. 

Sewage and other Waste Liquids. — The waste liquors from 
the different liming, bateing, puering, tanning, washing, and other 
soaking processes are, without any doubt, the most troublesome 
of any of the side-products which are obtained in the manu- 
facture of leather. In former times they were simply run into the 
nearest stream, but nowadays the various sanitary authorities and 



564 PRINCIPLES OF LEATHER MANUFACTURE 

other similar bodies will only permit comparatively pure waters 
to be turned into public streams or watercourses. 

Various methods of effecting the necessary purification of 
the waste liquors from tanneries have been proposed at different 
times, and have been used with -varying degrees of success. 
These methods may be divided into three heads : precipitation, 
followed by filtration or sedimentation ; land-treatment ; and 
bacterial purification. 

The first of these depends on the power of certain substances, 
such as alumina and oxide of iron, to carry down organic matter 
with them if precipitated in solutions containing it. The method 
usually consists in adding a sufficient quantity of lime to render 
the waste liquid slightly alkaline, and then treating it with 
some crude salt of aluminium or of iron, or vice versa. By this 
means a precipitate of aluminium or iron hydrate is formed, which 
encloses within itself a considerable proportion of the organic 
matter of the liquid, and after settling to the bottom of the 
precipitation-tank is drawn off as " sludge." Various chemicals 
are sold under fancy names, such as " alumino-ferric," " ferro- 
zone," etc., and have a composition not very dissimilar to that of 
crude sulphates of iron or alumina. In some cases by-products, 
such as the acid liquors used in preparing iron articles for 
" galvanizing," can be used with advantage. 

In the case of the waste liquors from a tannery, the use of 
these chemicals may often be avoided or reduced if sufficient 
care be taken in regulating the proportion of the various liquids 
which are to be mixed together and run into the settling-tank. 
As tanning matter combines with lime and dissolved hide-sub- 
stance to form a heavy brown insoluble precipitate, it is clear 
that if care be taken to have rather more waste lime-liquor mixed 
with the waste tan -liquors than is necessary to throw aU the tan 
out of solution, a very considerable amount of purification of the 
effluent will have taken place without any cost whatever to the 
tanner. Hence, if the proportion of waste lime is small in com- 
parison to that of the tanning liquors, an extra addition of lime 
may be necessary in order to precipitate the tannin. 

The precipitation- or settling-tanks are usually square or rect- 
angular vessels or pits, the size of which varies with the quantity 
of hquid to be treated, but of v/hich the depth rarely exceeds 6 
feet. They may be divided into two classes — the " intermittent " 
and the " continuous." In the former class the tank is fiUed with 
the mixed waste hquids, taking care that such a sufficiency of 
lime is present that the mixture is fairly alkaline to phenol- 
phthalein paper, and is then allowed to rest until the suspended 



WASTE PRODUCTS AND THEIR DISPOSAL 565 

matter has settled dqwn to the bottom of the tank, when the 
clear, or almost clear, upper liquid is drawn off, the remainder 
being the " sludge " ; some means must also be employed to pre- 
vent the passage of scum and floating matters. In the case of the 
intermittent process it is advisable to have two tanks, one of 
which is being filled while the other one is settling or being 
emptied. With the continuous process the liquids are run into 
the tank in the proportions calculated to give a maximum amount 
of purification, as described above, but as they enter very slowly 
the undissolved matter soon settles, and consequently the liquid 
may be continuously run out at the farther end of the tank. 
This plan, though it does not yield such good results in the hands 
of unskilled workmen, is yet useful in many cases, as only one 
tank is absolutely necessary. It is desirable that in running off 
the tanks the effluent should be taken as near the surface as 
possible by means of a hinged pipe attached to a float or some 
equivalent device, and care is required, as the tank gets low, to 
avoid the escape of any of the sludge. 

For continuous settling the tanks are usually long and some- 
what shallow rectangular ponds, into which the previously well- 
mixed precipitating liquid flows through a wooden trough fixed 
across one end and as long as the breadth of the tank, and per- 
forated with holes to allow the uniform and quiet influx of the 
liquid, which finally escapes by a similar trough crossing the 
opposite end of the tank. In front of the exit-trough a " scum 
board " must be placed, which is a simple plank dipping slightty 
below the surface of the liquid, so as to prevent any oil, scum, or 
other floating matter from passing out of the tank along with 
the clear effluent. Whether the intermittent or continuous 
system is employed, the effluent should in most cases be after- 
wards passed through a bacterial filter-bed, or treated by land 
filtration, before it is allowed to flow into a stream or river. 
Tannery effluents are usually received into sewers without further 
treatment than mixing and settling to remove solid matter, 
and many authorities are satisfied with the removal of merely 
such coarse suspended matters as might choke the sewers. Where 
continuous precipitation-tanks are used they must be emptied at 
frequent intervals, and the sludge run on to cinder-filters to part 
with most of its water. These filters are conveniently placed at 
a lower level than the settling-tanks, and it is generally necessary 
to return the effluent from them for further precipitation and 
settling. Several types of continuous settling-tank with upward 
flow have been devised by Mr Candy and others, which are very 
suitable for use where space is limited, but otherwise less costly 



566 PRINCIPLES OF LEATHER MANUFACTURE 

constructions are often sufficient. Apart from the question 
of obtaining an effluent sufficiently good to satisfy the sanitary 
authority, the treatment of the sludge is one of the greatest 
difficulties in the purification of effluents. It is usually very 
bulky, easily putrescible, and therefore difficult to dry ; it is 
of little value for manure ; and if allowed to remain long wet, 
its smell is very offensive. Chloride of lime is probably the best 
disinfectant. 

It has been mentioned that in most cases the liquid, and in 
every case the sludge, must be freed from solid undissolved 
matter by filtration. This may take place through open filters 
or through filter-presses. The open filters generally consist of 
a pit with an exit at the bottom for the filtered liquid. This 
pit is filled with either stones and sand, with clinker, ashes, or 
coke. Most tanners use clinker and ashes, as they do not cost 
anything ; and the material should be so arranged that while 
the lowest layers are very coarse, the surface of the filter-bed 
should be of the finest material. As soon as this has become 
covered with so thick a layer of solid matter that the filtration 
proceeds too slowly, the top surface of the filter may be removed 
with a rake (taking care to remove as little of the ashes or sand 
as possible), and burnt, or dried and used as manure. In some 
cases filter-presses are used which are composed of grooved or 
perforated plates with cloths between them through which the 
liquid is forced by pressure. The solid matter remains behind 
in the form of a comparatively dry " cake." The filter-cake, 
dried if desired, is sold as manure, for which it is in many ways 
very suitable, though its value as a fertiliser is not great. 
Although they work much more rapidly than do the open filters, 
the cloths so soon become rotten and have to be replaced, that 
the open ash-filter is on the whole the most convenient for the 
tanner's use. It will be readily understood that apparatus of 
this kind, though very efficient on a small scale, is quite out of 
the question when many thousand gallons of liquid have to be 
filtered daily, and so can only be effectively applied to " sludge." 

No system of chemical precipitation has as yet proved entirely 
satisfactory. Undoubtedly a great deal of purification is effected 
by this means, but in most cases the " purified " liquid is still 
too impure to be turned into a stream, though for various reasons 
this is often permitted by the authorities. 

A great advance was made in the purification of effluents 
when manufacturers were compelled by law to allow the effluent 
from the precipitation-tank to filter through land set apart for 
that purpose. In this case certain hardy cereals were sown on 



WASTE PRODUCTS AND THEIR DISPOSAL 567 

the land, which was watered as often as possible with the effluent. 
This latter, after soaking through the land, was drained off into 
the nearest stream. Although in many ways this treatment was 
satisfactory, it had the disadvantage of being very expensive, 
especially in the neighbourhood of large towns where the price 
of land is high, and, in addition to this, the conditions necessary 
for success were far from being correctly understood, so that the 
land often became " sewage-sick " or waterlogged, and ceased 
either to produce crops or to purify the effluent. It was not until 
the researches of bacteriologists proved that the purification by 
land-filtration was mainly due to the bacteria in the soil that any 
really satisfactory solution of the problem could be found, but 
the question has now been to a considerable extent simplified by 
the introduction of " bacterial treatment." 

Bacteria, considered from the point of view of their action 
on organic matter, are often classified as "anaerobic" and 
" aerobic," though many species are capable of existing under 
both conditions (cp. L.I.L.B., section xxiv.). The anaerobic 
bacteria thrive only in the absence of air, and their chemical 
action consists in breaking down the organic matter on which 
they feed into simpler, and generally more soluble, forms by pro- 
cesses which do not involve oxidation. The aerobic bacteria, 
on the other hand, require air or oxygen for their existence, and 
produce changes which are usually of a less complex character, 
but result in the complete oxidation and conversion of the organic 
matter to simple compounds, such as nitrates and carbonic acid, 
which are perfectly harmless and inoffensive. The two classes 
therefore are to a large extent complementary to each other, the 
anaerobic bacteria converting the animal or vegetable substances 
into more soluble and simple compounds which are adapted to 
the needs of the aerobic, which complete the destruction of the 
organic matter. 

In harmony with what has just been said, bacterial treatment 
of sewage is of two kinds, each of which may be used alone or 
in conjunction with a preliminary precipitation-process, but 
which are generally best used successively. The oldest form of 
bacterial purification depends mainly on the action of anaerobic 
bacteria, and is known as the " septic tank." This originally 
consisted of a tank sometimes filled with small pieces of coke, 
but generally containing the hquid only, and which was tightly 
closed to prevent access of air and escape of foul gases. It has, 
however, been found that if deep tanks (6 to 10 feet) are em- 
ployed, they soon become in continuous use so covered with 
scum and floating matter as effectually to prevent access of air 



568 PRINCIPLES OF LEATHER MANUFACTURE 

and light or any serious escape of smell. The liquid to be 
purified is allowed to flow very slowly through a tank or series 
of tanks of this description, entering about a foot below the 
surface through a distributing trough at one end and flowing 
out similarly at the other, at such a rate as to change the con- 
tents of the tank about once in twenty-four hours ; and when 
the tank is in working order, the liquid is much purified by the 
process, and most of the solid organic matter has become 
liquefied and disappears. It not unfrequently happens, especially 
where the septic-tank treatment is not very prolonged, that 
the liquid which escapes has a stronger and more offensive odour 
than it had on entering the tank. It is nevertheless really 
purer than before, the increased smell being due to the volatile 
products of the partially decomposed organic matter, and by 
passing the liquid through an open coke-filter the smell will be 
effectually removed. In all cases it must be borne in mind that 
as septic tanks and bacterial filters depend for their efficiency 
on the organisms they contain, time must be allowed for these 
to develop and accumulate before good results are obtained ; and 
for this about six weeks' use is generally necessary, after which 
they will continue to act for an indefinite period until they 
become choked by sand and inorganic matter. 

It must not be supposed that the action in the septic tank is 
wholly anaerobic ; and with weak sewage, most of the organic 
matter may under favourable circumstances be converted into 
nitrates and carbonic acid by this means only ; but generally a 
much more complete purification is effected by the subsequent 
use of "bacterial filters." These in their simplest form consist 
of tanks of about 4 feet deep, filled with coke, broken bricks, or' 
clinkers, and fitted with drain pipes at the bottom, by which 
they can be easily emptied.. These tanks, often known as 
" contact-beds," are filled with the sewage or septic-tank effluent, 
which is allowed to remain on them two hours, and the tank is 
then emptied, and allowed a rest of six hours for oxidation and 
aeration, during which considerable heat is developed, which pro- 
motes the bacterial activity. In most cases the sewage requires 
two such treatments, the last often through a bed with finer 
coke, in order to be completely freed from putrescible matter. 
In place of the intermittent process, as applied on the contact- 
beds, continuous aerobic filtration is often employed, the bed 
being so constructed as to allow of free admission of air at the 
bottom and sides, and the liquid to be purified being distributed 
on the surface by a sprinkler or some similar device, and allowed 
to trickle through the bed. The continuous process seems likely 



WASTE PRODUCTS AND THEIR DISPOSAL 569 

to supersede the intermittent one, as the beds are not only capable 
of treating a much larger quantity of sewage in proportion to 
their area, but are also less liable to choke. About six weeks is 
required, with either contact-beds or continuous filters, before 
the material they contain becomes coated with the necessary 
bacterial layer and they get into full working order. The results 
as regards the effluent are perfectly satisfactory, and the great 
difficulty and cost consists in the slow but inevitable choking of 
the beds, which involves the replacement of the porous material. 
This is considerably delayed by the use of a settled or precipitated 
sewage, and in this respect, beside its bacteriological function, 
the septic tank serves a useful purpose in settling insoluble matter, 
which is much more cheaply removed from it than from the filter- 
beds. It will be obvious that ordinary settling-tanks, if deep, 
fulfil many of the functions of the septic tank, and both lead to 
the production of a much more uniform liquid from the different 
effluents which the tanner produces, which is important in the 
subsequent bacterial purification. A good deal of interesting 
information on these subjects will be found in a paper by Mr W. H. 
Harrison on the " Bacteriological Treatment of Sewage."^ 

There are a good many patents in connection with the various 
methods of sewage purification, and some caution is necessary 
to avoid their infringement, though of course the general prin- 
ciples of settling and filtration, and the destruction of organic 
matter by bacterial action, are open to all. 

As a general rule the waste liquors from a tanyard or leather 
dye-works are exceedingly impure. They contain the organic 
matter (in a state of great putrefaction) from the soaks, bates, and 
puers ; other organic matter, also more or less putrefied, from the 
tan-pits ; the lime liquors, with their large proportion of lime 
and of dissolved protein, and in addition the various dyes and 
other chemicals which may have been used in the conversion of 
the raw hide into the finished leather ; and hence their efficient 
purification has presented difflculties which do not occur in most 
other trades. 

The different waste liquids are best run into a capacious tank, 
and, after being thoroughly mixed up together, are allowed to 
settle for some hours. By this means the greater part of the 
tanning matter will combine with the lime also present to form 
a heavy, brown insoluble substance ; some of the dye and other 
organic matter will become entangled in this, and thus be removed 
from the liquid. The clear liquid is next run off into a bacterial 
filter (preferably a septic tank, followed by an open coke-filter),. 
^ Journ. Soc. Chem. Ind.^ iQoo, P- 511. 



570 PRINCIPLES OF LEATHER MANUFACTURE 

and then into the nearest stream. If the tannery is near to a 
town, and the corporation sewers can be utihsed, it is probable 
that a filter made of spent tan may be substituted, as this material 
will not only remove all excess of lime from the liquid, but will 
also fix much of the colouring matter (Koenig). The tan, after 
being used for this purpose, contains so much lime in its pores 
that it is said to be useful as manure. 

In tanneries where large quantities of disinfectants, such as 
mercuric chloride, carbolic acid, etc., are used, it is necessary that 
the mixed liquids shall contain so much lime as to make them 
distinctly alkaline. In this way most of the disinfectants will 
be either precipitated or rendered inactive. Where arsenic is 
used in the limes it may be advisable to add a little ferrous 
sulphate (green vitriol or copperas) in order that the arsenic 
may form an insoluble compound with the iron, and so be removed 
along with the sludge. The ink produced by the action of the 
iron salt on the tan liquors will be completely removed by the 
bacterial filter. 

The problem of the utilisation of leather-scrap, shavings, 
moisings, and fluffing dust is a very difficult one. Although 
leather contains considerable quantities of nitrogen, its value as 
a manure is negligible, since it takes at least years to decay in 
the soil or to produce any useful effect. It is easily pulverised 
by scalding with steam or boiling water, drying, and grinding, 
but even the fine powder is almost imperishable. Probably 
better results would be obtained by treating it with strong, and 
if possible hot, sulphuric acid and incorporating it in super- 
phosphate mixtures, but, judging by the time it takes for solution 
in Kjeldahling, even this would not be very effective. It would 
also be decomposed by heat and alkalies, but in this case it would 
be difficult to prevent the escape of the ammonia formed. Prob- 
ably the best treatment is destructive distillation and recovery 
of ammonia from the escaping gases. In some experiments on 
condemned army boots 513 lb. of ammonium sulphate was 
recovered from a ton. The residual charcoal would also have 
decolorising and deodorising power, and there would be a good 
deal of combustible gas. 

Pulped leather has been used in mixture with vegetable fibre 
for making leather-board, but apparently the less proportion 
of leather the better the result. It seems curious, however, that 
some satisfactory way cannot be found of compacting pulped 
leather into stiffeners, heels, and various articles of that sort for 
shoe manufacture. 

A suggestion has been patented by S, Brough (1910) for the 



WASTE PRODUCTS AND THEIR DISPOSAL 571 

use of leather cut into small pieces, in conjunction with asphalt, 
bitumen, and limestone, for the making of more resilient roads, 
but, even if successful, the quantity available would not go far 
on the roads of the country. Possibly, however, a coating for 
concrete floors might be made on the same principle, which 
would render them warmer and more comfortable. Grease 
should be recovered from any leather containing it (pp. 559, 379), 
and means are described for stripping chrome leather on p. 572. ■■■ 

1 Compare M. C. Lamb, Journ. Soc. Chem. Ind., 1917, p. 986, and his 
patent for stripping with oxalic acid, E.P. 132,864. 



CHAPTER XXXII 

CONCLUSION 

It may be well in concluding this book to sum up shortly the 
most important of the advances which have been made since the 
publication of the first edition of 1903, some of which have been 
already discussed, while others are here alluded to for the first 
time in these pages ; and perhaps also to allow myself a few 
words of suggestion as to the opportunities for progress in the 
near future.^ 

Among the most important of the practical gains which are 
also of scientific importance may be mentioned Dr Stiasny's 
discovery 2 of a new class of synthetic tanning matters, the 
syntans, which can be produced so cheaply as to have already 
found extensive use in the tannery. From the scientific side 
these tans, of comparatively simple constitution, offer an oppor- 
tunity for the investigation of the relation of constitution to 
tanning effect which may throw much light not only on the 
nature of the vegetable tanning process, but on the causes of the 
quality we call astringency. 

Less important are the discoveries ^ of the effect of hydroxy- 
acids such as tartaric, which by forming complex ions with 
chromium renders possible the removal of chrome from chrome- 
tanned leather by Rochelle salt (sodium-potassium tartrate), 
and explains the effect of some of the products which occasionallj'' 
occur in chrome liquors reduced by organic matter, producing 
purple liquors which will not tan ; and the production of con- 
centrated chrome liquors by the direct reduction of strong 
solutions of sodium bichromate with sulphurous acid.'* 

An interesting new departure is also the vacuum tanning 
process of Mr Nance. All previous attempts to introduce tan 
into leather by the same use of vacuum as is so successful in 
creasoting timber (and there were many) had failed because the 

^ A portion of this chapter is taken from a lecture given by the writer 
to the Conference of the Leatherj Trade Federations at the Leathersellers' 
Hall, 17th November 1920. 

2 Ger. Pat. 262558, 191 1. 

^ Journ. Soc. Chern. Ind., 1916, p. 230. 

* Journ. Roy. Soc. of Arts, 66, 1918, pp. 747, 776. 

572 



CONCLUSION 573 

pores of the hide are filled with incompressible water instead of 
with air, and it was not until Nance used a vacuum so high that 
the water actually boiled in the pores of the hide at a temperature 
of only 70° or 80° F. that the water could be expelled and the 
entry of tan -liquor made possible. 

A new departure in drum-tannages is the use of viscous colloids, 
such as tragasol ^ and starch paste,^ as vehicles for the tanning 
extract, which allow it to be used in concentrated form without 
hardening the surface or drawing the grain, and probably them- 
selves contribute to filling the leather. 

Of more scientific interest, though perhaps of less immediate 
commercial importance, are Professor Meunier's investigations ^ on 
tannage with various chemical substances, and especially with 
quinone, which gives one of the most perfect and resistant leathers 
known, though its price precludes it at present from any extended 
use. Quinone is a derivative of benzol, a ring of six carbon atoms, 
to each carbon of which, in benzol itself, an atom of hydrogen is 
attached. In phenol (ordinary carbolic acid) the place of one 
of these hydrogens is taken by an OH or hydroxyl group, and 
the substitution of a second OH gives, as might be expected, three 
di-hydroxy-phenols, alike in composition, but differing in pro- 
perties, and in the place of the second OH on the ring. Taking 
the OH of common phenol as in the noon position, the two o'clock 
position gives catechol, the source of all the catechol tannins ; 
the four o'clock is resorcin, present in the dyestuff orcin ; and the 
six o'clock is hydroquinone, the common photographic developer. 
If hydroquinone be oxidised by exposure to the air or otherwise, 
the H's of the two hydroxyls combine with the O to form water, 
and the spare links of the deserted O's join hands across the ring 
and form quinone. Such a structure is naturally little stable, 
and in presence of hydrogen easily reverts to hydroquinone. 
In a I per cent, solution of quinone, pelt becomes first rose- 
coloured, then violet, and finaUy brown, and is converted into 
a soft tough leather which will stand boihng and washing with 
soap or even with dilute acids or alkalies, and which dyes readily 
with acid, basic, and mordant dyes. Meunier found hydroquinone 
in the used liquor, thus proving that the skin had not merely 
combined with quinone, but with oxygen. A similar tannage 
can also be produced by hydroquinone, but more slowly, and 
only in presence of air. The oxidation of the hide-substance is of 

^ Gum Tragasol Supply Co. Ltd., Hooton, near Chester. 
2 A. Turnbull and B. Carmichael, Eng. Pat. 101470, 1917- 
* Coll., 7, 1908, p. 195. Ibid., 8, 1909, pp. 58, 319. Ger. Pat. 206957, 
1909. 



574 PRINCIPLES OF LEATHER MANUFACTURE 

some theoretical importance. Meunier concludes that the quinone 
combines with the amino-groups of the skin, to which apparently 
acids and vegetable tannins also attach themselves. 

Meunier pursued his researches with other phenols and phenol 
derivatives, such as pyrogallol and gallic acid, and found that 
most of them would tan if exposed to the oxidising action of the 
air, and still more rapidly when in a slightly alkaline condition, 
which promotes oxidation. Gallic acid with access of air 
rendered gelatine insoluble in three' days, while gallotannic acid 
failed to do so in twenty-six days in a closed vessel, but rapidly 
did so when air was admitted. This seems to show that air has 
some function in tannage which has not been generally recognised, 
though whether the skin or the tannin, or both, must be oxidised 
is not yet clear. It may, however, be pointed out that if tannin 
will not tan gelatine in vacuo it will certainly tan skin, and the 
reason may possibly be that while the tannin permeates the hide 
and penetrates the extremely fine fibres, it is not able to penetrate 
gelatine in the mass, but only tans the surface, while the insolubili- 
sation of the interior is due to the oxidation of the gallic acid 
which is always present in commercial tannin or is formed by its 
decomposition. In experiments on the diffusion of tannins in 
gelatine jelly it has been found that this is apparently the case. 
Meunier's statement that gallic acid tans more rapidly in alkaline 
solution is noteworthy, and shows that its mode of action is 
radically different from that of the tannins, which will only tan 
in an acid medium. The gallic acid is, however, probably 
absorbed as an acid, though it only exerts its tanning effect when 
oxidised. The work throws an important and rather unexpected 
sidelight on the vexed question of the value of the phenolic non- 
tans, such as gallic acid, which are present in all tanning materials. 
Though partially absorbed by hide-powder in the ordinary 
analytical process, they certainly do not tan, and Wilson ^ has 
shown that they can be removed from it by washing, but it is 
clear that if exposed to the air in drying they will become fixed 
by oxidation, producing an actual tanning, and adding to the 
weight and solidity of the leather. These facts will have to be 
considered in any revision of the analytical method. The ideal 
analytical method of the future should give much more detailed 
information as to the constituents of the tan, but will consequently 
cost more in execution, for which users will have to pay, and the 
present process, imperfect as it avowedly is, has become of such 
commercial importance that no changes, even for the better, 
can be lightly undertaken. 

^ Journ. Amer. heather Chem. Assoc, 1920, p. 295. 



CONCLUSION 575 

Meunier/ following up the clue of oxidation, next experimented 
with such well-known oxidising agents as chlorine, bromine, and 
iodine, all of which precipitate gelatine from its solutions as 
insoluble compounds, and found that all were capable of con- 
verting skin into leather. Bromine, especially, in dilute solution, 
and with addition of salt to prevent undue swelling from the 
hydrobromic acid produced, speedily tanned the skin, which 
after washing with water, or more rapidly with a solution of sodium 
bisulphite to remove the surplus bromine, formed a white, 
supple, and finfe-grained leather of considerable toughness and 
resistance even to hot water, and which contained about o-g 
per cent, of bromine on the dry gelatine. Equally good and 
very similar results were obtained by the use of sodium hypo- 
bromite, formed by adding soda to the bromine solution till its 
yellow colour was just discharged, and in this case salt was not 
necessary, though the washing with water and bisulphite could 
not be omitted. 

As bromine is too expensive fot ordinary use it seemed de- 
sirable to experiment with the much cheaper chlorine, but 
gaseous chlorine or chlorine-water broke up and dissolved the 
gelatine before rendering it insoluble. Much better results were 
obtained with sodium hypochlorite [eati de javelle) and with 
bleaching powder, but more care was required than with bromine, 
and it was necessary to work at temperatures at or below io° C. 
(50° F.), which would be difficult in practice in summer, but 
Meunier suggests the use of the process for the temporary pre- 
servation of skins in place of pickling, or for their preparation 
for other tannages, and this might be practicable where very cold 
well-water was available. Ordinary " hypo " might no doubt 
be substituted for bisulphite for washing out the excess of 
hypochlorite. 

In all these cases the tannage appears to be wholly chemical, 
and no solid body which could coat the fibres is produced, so 
that neither Knapp's theory of coating the fibres nor the more 
modern ideas of adsorption can well apply. They give some 
colour, however, to a theory of Fahrion's as to the necessity of 
oxidation of the fibre itself. 

To complete his -investigations on the theory of tannage, 
Meunier next experimented with a purely physical tannage by 
dehydration and isolation of the fibres on the principle of Knapp's 
alcohol leather, 2 but instead of alcohol he employed solutions 
of potassium carbonate, which have a very high osmotic pressure 

1 " Le tannage au brome," Coll., 10, 1911, pp. 289, 373. 
^ Coll., II, 1912, pp. 54, 420, 



576 PRINCIPLES OF LEATHER MANUFACTURE 

and affinity for water. Placed in strong solutions, up to 80 per 
cent., water flowed freely from the skin under the osmotic pressure 
of the' solution, which seemed scarcely to penetrate it, and in a 
few hours left it in a state when after " putting out " with the 
sleeker, or even wiping with a cloth, it could be dried rapidly 
in the air, and after staking formed a soft white leather which 
was permanent so long as dry, but returned rapidly to pelt when 
soaked in water. As very little potassium carbonate is actually 
absorbed, there is a commercial possibility of using this process 
as a substitute for pickling. 

Another interesting set of experiments were made by repeating 
similar experiments in duplicate with potassium carbonate, and 
adding to one set 3 c.c. per litre of commercial formaldehyde 
solution. Where the potassium carbonate solutions were of 
less than 10 per cent, the leather of both series dried hard and 
horny, and more concentrated ones were necessary for good 
results. In each series the skin was equally leathered, but while 
those with the carbonate alone returned to pelt on soaking, those 
with formaldehyde when re-dried were as soft as before. Meunier 
concludes from this that the function of the sodium carbonate in 
the Payne and Pullman process is merely that of dehydration, and 
not that of an alkali. I am not myself quite convinced on this 
point, since their patent specifies only 1-9 to 2-8 per cent, of sodium 
carbonate, which is less dehydrating than the potassium salt, and 
in Meunier's experiments would be quite insufficient to produce 
a soft leather, while I can say from my own observation that 
Pullman's skins were perfectly leathered in the drum. It is, 
however, certain that satisfactory formaldehyde leather can be 
produced in the presence of other salts which are not alkalien. 
Meunier's opinion of formaldehyde as a tanning agent is not a 
very high one. He states that it slowly evaporates on exposure 
to air, leaving the leather hard and brittle, and that it can be 
removed by continued treatment with hot water, being in that 
respect very inferior to his quinone tannage. We have found at 
Leeds that formaldehyde can be quantitatively recovered from 
leather by hydrochloric acid so weak as decinormal. 

In connection with what has been said on new tannages, it is 
perhaps worth while to mention the accidental discovery of one 
in which I had a part. I had noticed in the laboratory the power- 
ful dehydrating effect of saturated solutions of ammonium 
sulphate, which easily produce a white leather like Meunier's, 
and I communicated this to Mr Seymour- Jones, thinking it 
might be useful in some cases where the use of acid in pickling 
is objectionable. He tried it on a practical scale, but used 



CONCLUSION 577 

commercial ammonium sulphate, which contains traces of tarry 
phenolic products, and found to his disappointment that the 
leather was permanent, and would not return to pelt. It is quite 
possible that by the choice of suitable tar-products a leather might 
might be made of commercial value. 

Leaving Professor Meunier, I must turn to the work of some 
other chemists. It was noticed by Liippo -Cramer ^ that colloidal 
silver peroxide precipitated in the gelatine film of photographic 
plates rendered it insoluble in hot water, and that it had the same 
effect on gum-arabic and starch. More recently Dr Erich A. 
Sommerhoff discovered that colloidal precipitates, such as 
hydroxides, sulphides, phosphates and silicates of the heavy 
metals, when drummed into pelt, rapidly produced complete 
tannage, and, what is much more surprising, that ultramarine, 
a fine and quite insoluble powder, had a rapid tanning effect. 
It has long been known that leather could be formed by the 
precipitation of such bodies in or on the fibre : Phosphate leathers 
have been commercially produced, and the " pyrotan " process 
owes at least part of its efficacy to a similar effect. Leathers 
have also been made, not only by chrome and alumina, but by 
other metals producing basic salts hydrolysed by the skin ; and 
the two-bath chrome leather owes its superior softness to that 
of basic tannage to the sulphur deposited on the fibre. Prob- 
ably almost any colloidal precipitate is capable of producing 
leather. 

Instances of this sort might be multiplied almost ad infinitum, 
but enough has been said to show that no single theory of tan- 
nage can embrace all possible cases, and that in most commercial 
processes more than one of these actions is involved. 

The investigations of H. R. Procter and his collaborators, 
amplified by many others, and especially by Dr J. Loeb, have 
been sufficiently described in other parts of this book and need 
not be discussed here, but it has much emphasised the importance 
of extended investigation of the region lying between ordinary 
alkalinity and acidity, roughly that between the reactions of 
phenolphthalein and of methyl orange, in leather chemistry, 
since within it lie the isoelectric points of gelatin and hide- 
fibre, on the one side of which these bodies act as acids and on 
the other as bases, and which is therefore the turning point in 
the reactions involved in tannage (p. ii8). It is obvious that 
this region cannot be investigated by the ordinary methods of 
titration, since it is mostly a question of the actual hydrion- 
concentration and " true " acidity, and not of the total con- 
1 Coll., 7, 1908, p. 24. 

17 



578 PRINCIPLES OF LEATHER MANUFACTURE 

centration of unionised acids, and quite different means must 
therefore be adopted. The most satisfactory of these methods 
is the electrometric, in which the actual acidity at the moment 
is determined by the electric potential of a voltaic cell, in which 
the positive element is a platinum plate saturated with hydrogen, 
but this method involves an elaborate apparatus and delicate 
manipulation which is not suited to the ordinary work of the 
tanners' laboratory. It is therefore of great practical importance 
that the invention of the " comparator " (App. D) has brought 
an approximate determination within the range of ordinary 
laboratory work, and at least made a great forward step in the 
determination of the swelling power of tanning hquors. Its 
effect in the control of liming and bating is not likely to be less 
than that with regard to the actual tanning, since the depleting 
effect of dehming is dependent on the closeness of the approach 
to the isoelectric point and the swelling power of limes on the 
concentration of ionised OH, just as that of liquors is on that 
of H+. 

While it is still hard to say whether tannage is in the main a 
" chemical " or a " physical " process, the work of the last twenty 
years has much emphasised the importance of the study of the 
colloid state of matter, and especially of the electric charges of 
the colloid particles on which their chemical action depends, 
and which usually change in sign at their isoelectric point. We 
are still in ignorance of the exact position of many of these 
isoelectric points, and their determination is a necessary work of 
the future.! More information as to the " dispersity " or size of 
the particles under different conditions of temperature and 
dilution is also urgently required, and in some important 
cases, as in that of chrome liquors, we do not yet know with 
certainty whether we are dealing with molecular or colloidal 
solutions. 

The valuable work of J. T. Wood on the bacteriology of bates, 
puers, and drenches was largely completed before the first edition, 
but since that time he has added largely to our knowledge of the 
enzymes concerned in these processes, and the trypsin bates have 
increasingly taken the place of those containing active bacteria ; 
and it is not likely that these wiU again come into use, though we 
may have to resort to bacteria for the production of the necessary 
enzymes (p. 219). The bacteriology and ferments of limes and 
liquors is however still to a large extent unexplored, and it is to 

1 The isoelectric point of gelatin is Ph = 4'7, and that of hide-fibre 
appears to be practically the same. See E. C. Porter, J.S.L.T.C., 
1921, p. 259. 



CONCLUSION 579 

be hoped that the new Research Institute of the Tanners' Federa- 
tions may be able to throw light on these important subjects. 

Mr Seymour- Jones's work on the histology of the animal skin 
is also a valuable addition to our knowledge, and we can only 
wish him health and strength to complete his promised book 
upon the subject. 

Since the first edition great i-mprovement has taken place in 
our analytical control methods, but the primary one of tannin 
determination has undergone little or no change, and though 
there seems little hope of superseding the present useful but 
empirical method, the time is approaching when its revision will 
be necessary, however difficult it is to make changes in one which 
has become of so great commercial importance. Mr J. A. 
Wilson's work has thrown new light on the large amounts of 
what are not strictly tannins which are absorbed by hide-powder, 
and without denying their uses to the tanner, it seems desirable 
that this should be reduced and made more constant, perhaps by 
a limited washing of the powder with addition of the washings 
to the non-tannin filtrate. The attempts to prepare a powder 
of absolutely constant absorptive power have proved largely 
abortive, and it is a question whether the origin of the trouble 
does not lie rather in the varying acidity of the liquors than in 
that of the hide-powder. At the same time it has been shown 
that a slight acidification of the liquor would greatly simplify 
the determination of the difficultly soluble tans by rendering them 
readily filterable, though at the same time probably increasing 
their amount. 



APPENDICES 

APPENDIX A 

THE DECIMAL SYSTEM 

The metrical system of weights and measures and the Centigrade 
thermometer scale have been generally used throughout the book 
as more international and scientific than the complicated systems 
still unfortunately in use in this country. Its much greater 
convenience in calculation, and the fact that it must ultimately 
come into use throughout the civiHsed world, and is already the 
only system in use in scientific laboratories, also make its com- 
prehension imperative. It has the advantage of being based 
on a single measure of length, the meter, from which all other 
measures of area and capacity are derived, so that instead of 
having to learn separate tables for each of these, it is easy to 
pass by a mental calculation of squaring or cubing from one set 
of dimensions to another. Thus the cubic decimeter is the hter, 
and a cubic meter is looo hters, and the liter of water weighs a 
kilogram, and the cubic meter a metrical ton or 2204-6 English 
lbs., and if we know the specific gravity of a body, its weight in 
kilograms is at once apparent. Thus a stone of Sp. Gr. 3-0 and 
I meter cube is at once seen to weigh 3 metrical tons, and for 
most aqueous liquids the weight does not vary seriously from 
I kilo for each liter. 

The following table gives the figures required for reduction 
from the English to the metrical system, or vice versa: — 

I meter =39'37 inches. . 

I millimeter = 0-003937 inch. 

I liter = 0-2202 gallon. 
I cub. cm. water= 15-432 grains. 

I cub. meter =35-317 cub. ft, 

I foot = 0-3048 meter. 

I inch =25-34 millimeters. 

I gallon = 4'54i liters. 

I grain =64-8 milligrams. 

1000 cub. ft. =26-314 cub. meters. 

Actual reduction is, however, generally unnecessary if the 

5S0 



THE DECIMAL SYSTEM 



581 



question be treated as one of proportion. Thus a solution of 
I gram per liter is of the same strength as one of i lb. per 100 
gallons (1000 lb.), and very approximately, as one of i oz. avoir- 
dupois per cubic foot (i cubic foot weighs 9971 oz.). In the case 
of pits, it is often simplest to measure them directly with a meter 
rule ; length, breadth, and depth, measured in decimeters and 
multiplied together, giving the contents in liters, and, in the case 
of water, the weight in kilograms. The capacity of a rectangular 
tank is length x breadth x depth, that of a cylindrical one is 
(half-diameter) 2 X depth X 3-1416, or approximately by 3^ : 
the Imperial gallon is 277-274 cubic inches, and that of water 
weighs exactly 10 lb. at 62° Fahr. The American gallon is the 
old English wine gallon, and contains only 231 cubic inches. ^ 

The Centigrade or Celsius thermometer divides the difference 
between the freezing and the boiling points of water into 100°. 
The following table gives the points at which its scale agrees 
without fractions with that of Fahrenheit : — 



Comparison of Centigrade and Fahrenheit Degrees 
°F 



°C. 
—20 

-15 

— 10 

- 5 



- 4 

- 5 
14 
23 



°C. °F. 


°C. °F. 


X. °F. 


°C. °F. 


°C. °F. 


32 


20 68 


40 104 


60 140 


80 176 


5 41 


25 77 


45 113 


65 149 


85 185 


10 50 


30 86 


50 122 


70 158 


90 104 


15 59 


35 95 


55 131 


75 167 


95 203 



°C. °F, 
100 212 
105 221 
no 230 
115 239 



^ The avoirdupois pound is 7000 grains and the ounce 437^^ grains. 
The troy and apothecaries' pound is of 5760 grains, and the ounce (in 
which all precious metals are weighed) is 480 grains, but the " fluid ounce " 
of water is only 437I grains. In chemistry the "Mohr's liter" (i kilo 
of water at 15° C.) is generally used in place of the true liter of i kilo 
at 4° C. 



APPENDIX B 

ORIGINAL PAPERS ON THE GELATINE EQUILIBRIUM 

As most readers dislike (and shirk) mathematics, and it is yet im- 
possible thoroughly to understand a mathematical subject without 
it, it has seemed best to print two of the most important original 
papers in full, rather than burden the text with a long mathe- 
matical explanation. The selection has been made on several 
grounds. The first paper was originally published in German, 
and though it has since appeared in English in the J.A.L.C.A., 
there are many readers to whom it is not readily accessible. It 
contains a mass of experimental work, much of which has been 
utilised in subsequent papers, and for a full understanding of 
any question it is important to know something of its historical 
development ; and it is indeed the want of this which leads many 
unscientific people to complain of the changeableness of scientific 
views, when, with more knowledge, they would realise that the 
newer view is simply the logical and necessary outcome of the 
older in the light of wider knowledge. Though this first paper 
left much of the problem unsolved, it is interesting to its writer 
to see how much of the later solution is suggested ; and the final 
answer was only rendered possible by the important papers of 
Professor Donnan on " membrane equilibria, " which were 
published in the same year. A curious proof of the scientific 
foundation of the theory is found in the determinations of " acid 
fixed " which are given in Tables III. and IV. and shown in fig. 
123, and which produced a peculiar curve which at the time 
seemed inexplicable. The explanation could not be given till 
T914, when an equation to the curve was published in the Trans, 
of the Chemical Society, p. 325, and its exact parallelism with the 
experimental results was most striking. The rather important 
paper [Trans. Chem. Soc, p. 313) in which this appeared is not 
here reprinted, as it is pretty readily accessible, and, although 
it marked a great advance at the time, the theory has been con- 
siderably amplified and perfected in the paper of 1916 by Procter 
and Wilson, which is given. 

A list of some of the more important papers on the acid- 
gelatine equilibrium are given : — 

582 



PAPERS ON GELATINE EQUILIBRIUM 583 

H. R. Procter, " Action of Acids and Salt-solutions on 
Gelatine," Journ. Amer. Leather Chem. Assoc, 6, 1911, p. 270. 

H. R. Procter, " Equilibrium of Dilute Hydrochloric Acid 
and Gelatine," /oMr«. Chem. Soc, 105, 1914, 313. 

Procter and Wilson, " Acid-gelatine Equilibrium," Journ. 
Chem. Soc, 109, 1916, 307. 

Procter and Wilson, " The Swelling of Colloid Jellies," 
Journ. Amer. Leather Chem. Assoc, 11, 1916, 339. 

J. A. and W. H. Wilson, " Colloidal Phenomena and the 
Adsorption Formula," Journ. Amer. Chem. Soc, 40, 1918, 886. 

And other papers, particularly those of J. Loeb in Journ. Gen. 
Physiology, 1918-1921. 

PART I.— ON THE ACTION OF DILUTE ACIDS 
AND SALT-SOLUTIONS UPON GELATINE 1 

By Henry R. Procter 

The investigation which forms the subject of the following 
paper was begun in 1897, and much of the experimental work 
was done in 1898 in conjunction with Mr Richard Paget, who, 
it was hoped, would share in the authorship of the paper. Cir- 
cumstances, however, prevented its completion in co-operation, 
and the work has been carried on at intervals, with various 
assistance, up to the present time ; and although there remain 
many points stiU unsolved, it seems desirable, in consideration 
of the present interest in colloid questions, no longer to delay the 
publication of what is already completed. 

The investigation was originally undertaken in the hope that 
the study of comparatively simple cases of colloidal swelling and 
contraction might throw some light on the complicated pheno- 
mena of the tanning process, and especially on the very curious 
results ol the treatment with acid and salt known as " pickling," 
and on the mineral tanning processes in which acids and salts 
are employed ; but as it proceeded it became obvious that much 
wider and more important scientific issues were involved, which 
included the whole theory of colloid swelling. 

The pickling process consists in principle in treatment of the 
skin with a very dilute bath of sulphuric acid, in which the con- 
nective-tissue fibres are strongly swollen ; and subsequent im- 
mersion in a concentrated solution of common salt, in which not 
merely the swelling disappears, but the fibres become greatly 
dehydrated, and the skin converted into a kind of leather. As 

1 This article was published in German in Kolloidchemische Beihefte, 
Bd. II., Heft 6-7, 191 1. It also appeared in the 7.^. L.C./i., 6, 191 1, p. 270. 



584 PRINCIPLES OF LEATHER MANUFACTURE 

even saturated solution of common salt has no dehydrating effect 
without the preliminary acid treatment, the effect is at first sight 
striking and unaccountable. 

The swelling action is complicated in skin by its anatomical 
structure, which allows it to absorb liquids not merely by colloidal 
swelling, but capillarily in the interstices between the fibres, and 
it was obvious that no quantitative study could be made unless 
means were devised to separate the two effects. Fortunately, 
however, gelatine behaves in a manner at least qualitatively 
identical with hide-fibre, and the very close chemical relationship 
between the two justifies the assumption that the same chemical 
affinities are involved, while from the absence of structure 
capillary absorption is excluded. Comparative experiment con- 
firmed this anticipated identity of behaviour ; and as the experi- 
ments were only intended as a preliminary investigation, ordinary 
commercial thin sheet gelatine was selected as a material. For 
the same reason, and to avoid complicating the work, slight 
variations of laboratory temperature, and >other secondary dis- 
turbing causes such as adhering moisture, were neglected, and a 
method of experiment adopted which was capable of compara- 
tively rapid execution. Sheets of thin French gelatine of the 
purest kind were cut, air-dry, to portions of about i gram in 
weight, and soaked in the requisite solutions, their gain in weight 
determined after draining as far as possible from adhering 
moisture, and both the gelatine and the residual solution analysed 
as regards acid and salt, and the whole calculated to ash-free 
gelatine dried at 110°, and to milligram-molecules per gram. The 
air-dry gelatine in the earlier experiments contained 16-07 P^r 
cent, of moisture, which, as it was kept in a stoppered bottle, was 
practically constant ; and i-ig per cent, of ash consisting mainly 
of lime with traces of sulphites and phosphates. It was, no 
doubt, a bone-gelatine. For all the earlier experimental work the 
same sample of gelatine was used, but for some later series Of 
determinations other gelatines were employed of the same 
character but not actually of the same parcel. This may account 
for some variations between different series of experiments, while 
any single series gave as a rule very consistent curves. 

The Swelling of Gelatine in Water 

The extent to which a gelatine will swell in cold water at a 
given temperature is to a great extent a specific quality of the 
particular sample, influenced by the proportion of partially 
hydrolysed gelatine-products which are always present in the 



PAPERS ON GELATINE EQUILIBRIUM 585 

commercial article. , These indeed cannot wholly be avoided, since 
they are formed to some extent whenever the gelatine jelly is 
heated so as to melt it, and they are unquestionably the main 
cause of those variations in character which have been attributed 
to what has been called the " Vorgeschichte " (previous history) 
of the jelly. Traces of soluble electrolytes also affect it osmo- 
tically, and perhaps chemically. That gelatine and other 
gelatinising substances do not swell to infinity and become 
colloid solutions like gum and dextrine is due to the sohd but 
elastic structure which is formed at setting, the cohesion of which 
finally balances the attraction of the gelatine for water. Under 
these circumstances it seemed not improbable that the swelling 
maximum of any given jelly would be influenced by the volume 
of its structure at the moment of setting ; and this is proved to 
be the case by the following experiment. 

Solutions containing approximately 5, 10, and 20 per cent, of 
air-dried gelatine were cast in glass tubes on wire spirals for 
convenience of handling, and were then dried for some days in a 
current of dry air, weighed, and allowed to soak in water at 
laboratory temperature, and weighed at intervals. Taking the 
weight of actual dry gelatine as unity, the amounts of water 
absorbed were as follows : — 

Table I. — Absorf 



Dried in air 
After soaking 24 hours 
After soaking 72 hours 
After soaking 96 hours 
After soaking 120 hours 
After soaking 144 hours 
After soaking 168 hours 
In original jelly 

As will be seen from the figures, the original setting volume has 
considerable influence on the maximum swelling, but is evidently 
not the sole determining cause. 

Action of Alcohol on Gelatine Jelly 

It is well known that the swelling of gelatine jelly can be 
reduced by treatment with alcoholic solutions, and with absolute 
alcohol it becomes a hard and apparently dry mass. As there 
is no reason to suppose any chemical action of alcohol on gelatine. 



J OF Water 


BY Gelatine 


5 


10 


20 


per cent. 


per cent. 


per ce 


o-i 


o-i 


0-2 


. 8-4 


4-3 


37 


117 


6-0 


5-1 


12-6 


6-5 


5-4 


13-2 


6-9 


5-5 


. 13-6 


7-2 


57 


14-6 


77 


5-8 


23-2 


ii-i 


5-0 



586 PRINCIPLES OF LEATHER MANUFACTURE 

which on soaking in water returns to its original jelly-condition, 
the case seems a favourable one for the study of the effect of 
purely physical forces on jellies. Gelatine is practically quite 
insoluble in cold alcohol either pure or dilute, and conversely, 
even quite weak jellies are semi-permeable to alcohol in solution. 
Alcohol placed in a porous cell lined with gelatine, and immersed 
in water, develops a considerable osmotic pressure, and masses 
of gelatine jelly dehydrated by alcohol absorb scarcely any of 
the latter. 

In order to get some idea of the effect of alcohol upon swell- 
ing, weighed portions of air-dried thin sheet gelatine were im- 
mersed in a series of mixtures of alcohol and water for twenty-four 
hours, and again for twenty-four hours in renewed portions of the 
same solutions. This length of time had been found sufficient in 
previous experiments to establish practical equilibrium. The 
portions were then drained and weighed to determine the swell- 
ing, the gravity of residual alcohol taken, and its percentage 
calculated by the ordinary tables, and the pieces dissolved in hot 
water, and distilled to a volume of 25 c.c. of distillate, of which 
the gravity was taken to determine alcohol in the gelatine. Only 
in the case of the 100 per cent, alcohol did the gravity of the 
distillate fall so low as 0-999, and in this case only to 0-9979, 
so that any alcohol found may very well have been that merely 
adhering to the surface of the gelatine, and more exact methods 
of experiment must be adopted before conclusive evidence of any 
solubility of alcohol in gelatine jelly can be obtained, though it 
seems possible that when the gelatine is nearly dehydrated, some 
alcohol is absorbed. 

A second series of experiments were also made in a similar 
way, in which the gelatine was swollen in water for twenty-four 
hours before treatment with the alcoholic solutions. The results, 
which are given below, are almost precisely similar to those of the 
first series, except that in the 90 per cent, and 100 per cent, 
alcohol complete equilibrium does not appear to have been 
reached. No evidence of penetration of the alcohol into the jelly 
was obtained, the gravities of the distillate ranging from 0-9996 
to I -000. The equilibrium appears to be completely reversible. 

Table II. gives the weight of swollen gelatine obtained from 
I gram of dry. It will be observed that the curve is quite a 
regular one. The weight of the gelatine from absolute alcohol 
is slightly less than its weight air-dried. 

It is of course impossible to calculate theoretically the osmotic 
pressures of alcohol in such concentrated solutions as were here 
used, but the curve is such as would be expected from osmotic 



PAPERS ON GELATINE EQUILIBRIUM 587 












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588 PRINCIPLES OF LEATHER MANUFACTURE 

pressure acting on a material with a certain elastic rigidity of its 
own ; and there seems no reason to invoke other forces. Prob- 
ably if the osmotic pressures of the alcoholic solutions were 
independently determined, such experiments might furnish a. 
means to calculate the elasticity of the jelly. Neglecting any 
small attraction of the alcohol for the gelatine, the equilibrium 
is probably 
Cohesive attraction of jelly ] 

+ ^= attraction of gelatine for water. 

Attraction of alcohol for water] 

This is of course merely an inverse way of stating the osmotic 
view, since osmotic pressures are " partial " kinetic pressures, 
and may be expressed as " minus internal pressures " of the 
solvent ; just as in a mixture of air and water-vapour at atmo- 
spheric pressure the pressure of air in the mixture is lowered by 
that of the water-vapour, which is equivalent to a minus atmo- 
spheric pressure. 

Alcohol, though it precipitates hot gelatine solutions when 
added in large quantity, can be mixed in moderate proportions 
without causing separation, and the mass sets to an apparently 
homogeneous jelly, which, if alcohol is insoluble in jelly, must 
really consist of alcohol-water solution of such concentration 
as corresponds to the equilibrium just discussed, enclosed as an 
emulsion in a jelly medium. Such an emulsion should swell 
more in water than a plain jelly, since not only will the jelly 
absorb all the water necessary for its maximum swelling, but 
the alcoholic emulsion-globules will become diluted, and exert 
an outward pressure on the jelly mass. On the other hand, if 
the action of the alcohol were a chemical one, lessening the 
absorptive power of the gelatine, the swelling should be reduced 
whether the alcohol were introduced from without, or w^re 
already present in the mixture. Experimentally, it was found 
that of two somewhat concentrated jellies of equal strength, 
one made with water alone, and one with a mixture of water 
and alcohol, the latter swelled much the more, thus confirming 
the emulsion-character of alcoholic jellies. It is almost certain 
that such jellies would show microscopically the cellular struc- 
ture which has been attributed by Biitschli and van Bemmelen 
to jellies in general. 

The Action of Acids on Gelatine 

It is well known that gelatigenous fibre is swollen by all dilute 
acids which are sufficiently ionised, although very feeble acids 



PAPERS ON GELATINE EQUILIBRIUM 589 

such as boric, carbonic and sulphydric, have Httle or no swelHng 
effect, and the same is true of many of the weaker organic acids. 

Gelatine is similarly affected. A gelatine which absorbs seven 
or eight times its weight of pure water may absorb over fifty times 
its weight of very dilute hydrochloric acid. For the most de- 
tailed experimental work hydrochloric acid was chosen, as a 
highly ionised and. tj^pical monobasic acid, which could be easily 
estimated both acidimetrically and by silver nitrate. A further 
reason for the selection was that although in the commercial 
pickling process already mentioned sulphuric acid is used in con- 
junction with excess of common salt, yet the acid principally 
active must necessarily be hydrochlori-c ; and as a satisfactory 
pickling can be produced by this and salt alone, nothing could be 
gained as regards principle by complicating the equilibrium with 
the presence of sulphuric acid and sulphates. 

The general method of experiment was similar to that which 
has been already described. Pieces of air-dried sheet gelatine of 
about I gram in weight, of which the content in dry ash -free 
gelatine was known, were soaked in solutions of acid of known 
volume and concentration for forty-eight hours, which was found 
a sufficient time to produce a steady equilibrium. The volume 
and strength of the residual solution was determined acidimetri- 
cally with standard KOH solution and phenolphthalein, the 
swelling of the jelly was measured by weighing after draining, 
and it was subsequently melted and the absorbed acid similarly 
titrated, it having been proved by preliminary experiments that 
the whole of the acid present could be thus determined, and that 
no difference in result was caused by melting the gelatine. Any 
slight variations from this procedure are noted in connection 
with special series of experiments. 

In an early series of experiments it was found that though the 
whole of the acid present in the jelly was estimated using phenol- 
phthalein as indicator, yet only a portion was determined when 
methyl orange was used, although to free hydrochloric acid both 
indicators are equally sensitive. It is therefore clear that in the 
jelly a portion of acid is combined either chemically or by adsorp- 
tion, in such a way that it is less ionised as regards H-ions than 
the remainder which behaves as if merely dissolved in the jelly, 
and of course varies with the degree of swelling. In' order to 
get rid of the complication thus introduced, it was assumed as a 
first approximation that the absorbed volume of liquid was of 
the same concentration as that of the surrounding acid solution, 
and that the excess which was always found on the titration with 
phenolphthalein was " fixed " or more closely combined with the 



590 PRINCIPLES OF LEATHER MANUFACTURE 

gelatine. This " fixed acid " proved to be usually somewhat 
lower but roughly approximate in quantity to that estimated by 
phenolphthalein but not by methyl orange, and obviously re- 
presents the excess of acid absorbed by the gelatine, though it 
does not accurately determine what portion is attached to the 
gelatine and what to the absorbed water, and it wiU be shown 
later that the quantity of acid really fixed by the gelatine is 
greater than that so determined. Still it affords a ready means of 
comparing the character of the absorption, and as such, is given 
in the tables. 

Table IIL represents the results of more than one series of 
experiments, the Roman numerals of the first column indicating 
the series. These experiments were made in 1899 and 1900 on 
one sample of French gelatine, the titrations being done mostly 
by Mr Paget. Table IV. represents experiments made more 
recently, with slight variations of method suggested by experience, 
on another sample of gelatine of a slightly more acid character, 
and with apparently a greater solid cohesion, as the maximal 
swelling is in all cases less, though the character of the curves 
9.nd the position of the maxima in general show good agreement 
with the earlier results. Especially in the lower concentrations 
of Table IV. some ambiguity is caused by the acidity of the 
gelatine, consisting mainly of bisulphites, which amounted to 
0-282 mgr.-mols. per gram as indicated by phenolphthalein, but 
did not affect methyl orange, and of which at least a portion 
diffused into the outer solution, and affected its molecular acidity 
as determined by phenolphthalein, but could hardly have much 
influence on the acid " fixed." Determinations of the strength 
of the outer solution by methyl orange, which certainly represent 
the whole of the acid present as HCl, are therefore given, with 
corresponding calculations of " fixed " aci4, and of the acid 
absorbed in the gelatine as determined by the two indicators. 

In reference to this work attention must be drawn to a paper ^ 
by Dr Wolfgang Ostwald on the swelling of gelatine, in which 
he gives curves for the swelling of gelatine plates in acids and 
alkalies of different concentrations. In both cases he shows the 
existence of a maximum such as has just been described, but he 
also observed a minimum with very dilute solutions, of which the 
present writer has found no trace.^ Ostwald himself ascribes 

1 Wo. Ostwald, " Ueber den Einfiuss von Sauren und Alkalien auf die 
yuellung der Gelatine," Archiv fiir die ges. Physiologie, Bd. 108. Bonn, 1905. 

^ This minimum has since been found at the isoelectric point Pjj=4-7, 
or N/5 0000, but could not well be detected by the means of titration 
used. (H. R. P.) 



PAPERS ON GELATINE EQUILIBRIUM 591 

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592 PRINCIPLES OF LEATHER MANUFACTURE 

this tentatively to the originally acid reaction of the gelatine, 
and in this he is probably correct, as the acidity is usually due to 
bisulphites, which are acid to strong bases and basic to strong 
acids, and of which neutralisation in either sense would 
probably diminish the original sweUing in water. The plates 




Fig. 123 (Tables III. and IV.). — Abscissae; mgr.-mols. HCl in i grm. solution. 
Ordinates ; + = results from Table III., * = results from Table IV. 

he employed were much thicker (3 to 4 mm. as compared to about 
0-25 mm.) than those used in the present research, so that it is 
doubtful if real equihbrium was reached. 

Table V. gives the results of a series of experiments intended 
to test the reversibility of the equilibrium gelatine-acid-water. 
The gelatine was swollen for forty-eight hours in a solution which, 
when equihbrium was reached, was of 0-2253 mols. per mil con- 
centration, and then for twenty-four hours in solutions of varied 



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PAPERS ON GELATINE EQUILIBRIUM 593 

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594 PRINCIPLES OF LEATHER MANUFACTURE 

(lesser) concentration. The latter time does not seem to have 
been quite sufficient for equilibrium to be again attained, but there 
is no reason to suppose the absorption as other than completely 
reversible. The lowness of the " fixed " acid as calculated by 
deducting the calculated acid of the solution absorbed is some- 
what remarkable, and it is unfortunate that in this series no 
parallel determinations were made with methyl orange. Several 
possible explanations may be suggested, but it seems best to defer 
discussion for further experimental investigation. The portions 
which were only treated for forty-eight hours in one solution are 
normal in respect of " fixed " acid. 

Although no exact quantitative result can be expected from the 
somewhat crude method of experiment, it is evident that graphic 
plotting of Tables III. and IV., fig. 123, represents the curves of a 
regular equilibrium, the figures being fairly consistent for any one 
series of determinations, and the errors of experiment not greater 
than may be expected, considering the influence of various un- 
determined factors, such as the cohesive elasticity of the jelly, 
and the extremely small forces involved in considerable changes 
of volume near the swelling maximum. The approximately 
horizontal course of the " fixed " acid line, after a certain con- 
centration of acid is reached, strongly suggests the idea of a 
definite though hydrolysing chemical compound, rather than a 
merely physical one ; and the determinations of " fixed " acid 
by the difference of reaction of phenolphthalein and methyl 
orange prove a decided change of concentration of the H' ion at 
or near that particular point. The occurrence of a very marked 
maximum of the swelling volume (S) is striking, and this follows 
naturally from the combination of the two regular curves of 
total acid per gram of gelatine (a) and of " fixed " acid (/"), 

since by the mode of calculation S= -, x being the concentra- 

X 

tion of the external solution given in column a. 

As regards other acids, only a limited amount of work has 
been done. With weak acids like acetic and lactic, no definite 
maximum of swelling has been observed,^ the absorption of liquid 
increasing with the concentration till solution of the jelly begins. 
With formic acid a maximum occurs at a concentration of about. 
0-07 gram-mols. per liter, but it is less marked, and the rise to it 

1 It has been since observed by Mr Atkin {J.S.L.T.C., 1920, p. 187) 
that this occurs with all acids at a Ph of 2-4 of the external acid which 
corresponds ver}' closely with the Ph = 3'o of th^ gelatin itself found by 
Loeb, but this H- concentration was not reached with the very weak 
acids. (H. R. P.) 



PAPERS ON GELATINE EQUILIBRIUM 595 



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596 PRINCIPLES OF LEATHER MANUFACTURE 

is much more gradual than in the case of hydrochloric acid. 
Owing to the fact that no repression of swelling takes place 
with the weaker acids, a greater total swelling can be obtained 
by concentrating the solutions with acetic and probably with lactic 
acid than with hydrochloric, though it is evidently accompanied 
with greater solution, and possibly by structural changes of the 
gelatine. 

As regards the fixation of acid, the methyl orange method is 
inapplicable to any but the strongest acids, but calciilating the 
absorbed solution as of equal strength to the external, figures for 
the excess-absorption are obtained which for acetic acid of 
medium concentrations are somewhat lower, and for lactic and 
formic about the same or slightly higher than those for hydro- 
chloric. At the higher concentrations, the total absorbed acid 
■s so large that experimental error makes the determination of 
" fixed " acid irregular and unreliable, and from the lowest con- 
centrations the value rises to a fixed average much more gradually 
than in the case of strong acids. 

As regards sulphuric acid, but few determinations have been 
made, but these show that it produces a maximum swelling effect 
at a low concentration of which the value has not yet been deter- 
mined, and the apparently " fixed " acid is also somewhat larger. 
No experiments have yet been made on the determination of 
" fixed " acid by methyl orange, but as the change of colour of 
this indicator is gradual, it is evident that determination by mere 
titration is somewhat rough, and it is proposed to investigate the 
subject further by the actual determination of ionisation-constants. 

The following tables VI. to IX. give the results of the work 
which has already been done on acids other than hydrochloric. 

Table X. (XXIX. 1-3) gives a few determinations on sheep- 
skin and shows that a maximum exists, the weaker solution swell- 
ing more than the stronger ; and the acid " fixed " is very similar 
in amount to that fixed by gelatine. The skin was unwooUed in 
the customary way, freed from lime, and dried at 80° C, and 
soaked in water till soft, before use. 

Attention must here be drawn to a research published by 
Stiasny on the absorption of water and acid by hide-powder and 
ox-hide ^ by quite different methods to that adopted by the writer. 
From the data given it would be difficult or impossible to calculate 
the acid fixed, but the occurrence of a maximum of swelling in the 
weaker solutions is in both cases very clearly marked. The 

^ Stiasny, " Ueber negative Adsorption, und die Bestimmung der 
Schwellwirkung von Sauren auf Hautpulver und Blosse," Gerber, 1909, 
pp. 183 et seq., and Collegium, 1909, pp. 302 et seq. 



PAPERS ON GELATINE EQUILIBRIUM 597 



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results with hide-powder and ox-hide showed considerable diver- 
gence, which Stiasny attributes to difference in texture, but which 
perhaps may have been partly due to the time given having been 
insufficient to establish complete equilibrium. 

One of the most striking effects of the " pickling process " which 
gave rise to the investigation is the extraordinary dehydration 
produced by the action of strong solutions of common salt on 
the acidified skin fibre, and also on the acidified gelatine ; and 
acidified gelatine is also precipitated by it as a coherent mass from 
its warm solutions. No such effect is produced by common salt 
alone on neutral fibre or gelatine, the effect even of saturated 
solutions being somewhat to increase the swelling — a gelatine 
absorbing about eight times its weight of water being capable of 
taking up about eleven of saturated common salt solution, and a 
larger quantity of one of medium dilution. The results of a 
series of experiments are given in Table XL Some other salts, 
however, and notably ammonium sulphate and some other sul- 
phates, are well known to exercise a powerful dehydrating effect 
on swollen gelatine or skin, and even to precipitate gelatine from 
strong warm solutions as a coherent mass. The discussion of 
these actions of neutral salts must be deferred till the effects 
of salts in acidified solutions has been more fully considered. 
It may be pointed out, however, that the effect is most notice- 
able in the case of sulphates of weak bases, such as ammonium 
and zinc. 

It is unimportant whether the gelatine or skin-fibre is first 
swollen by acid and then submitted to the action of salt solution, 
or the proceeding is reversed, since similar effects are produced 
by the addition of a suitable quantity of acid to the already salted 
gelatine, and an effective pickling may be produced by adding 
a calculated quantity of acid to skins placed in a strong brine, 
although commercially the method is more costly. The quantity 
of acid which is most effective is larger than that required to 
produce a maximum swelling, since the presence of salt enables 
the skin or gelatine to " fix " a larger quantity of acid than it can 
do in an equally dilute acid solution without salt. In presence 
of sufficient salt, however, the necessary quantity of acid may be 
(and commercially usually is) largely exceeded without much 
interfering with the result, though large excess is undesirable, 
and in dilute salt solutions diminishes the dehydration. Some 
instances of the action of acidified salt solutions on skin are given 
in Table X. (XXX. and XXXI.), and the results of much experi- 
mental work on gelatine in presence of hydrochloric acid and salt 
in different proportions in Tables XII. and XIII. 



PAPERS ON GELATINE EQUILIBRIUM 60 1 

Comparing the results with those of Tables III. to V. which give 
the results with hydrochloric acid alone, it wiU be noted that 
the total acid absorbed by i grm. gelatine is lower, the con- 
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6o2 PRINCIPLES OF LEATHER MANUFACTURE 



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PAPERS ON GELATINE EQUILIBRIUM 603 
Table XIII. — Gelatine, HCl and NaCl. Acid Varied, Salt 

APPROXIMATELY CONSTANT. ThE AmOUNT OF AciD IS 

determined after Equilibrium is established, that 
OF Salt is calculated on the Original Solution, as 
Fixation is negative and negligible as compared to 
THE Total Concentration. 





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1-52 


1-228 


0-567 


0-661 


I-I2I 


— 0-064 


3 


o-o6oi 


1-3 


1-257 


0-554 


0-703 


I-175 


+0-354 


4 


0-0410 


1-64 


1-199 


0-514 


0-685 


1-132 


-0-329 


5 


0-0251 


2-05 


1-138 


0-438 


0-700 


1-087 


-0-545 


6 


0-0126 


2-05 


I-I02 


0-378 


0-724 


1-076 


—0-762 




Salt in 


original 


solutions 3-02 mols. per gram. 




VII. I 


0-1796 


1-54 


1-508 


0-592 


0-916 


1-232 


— 0-912 


2 


0-1492 


1-58 


1-434 


0-548 


0-886 


I-I98 


-0-853 


3 


0-1184 


1-52 


1-381 


0-565 


0-816 


I-20I 


-0-885 


4 


0-0872 


1-60 


1-303 


0-665 


0-638 


1-163 


— 0-620 


5 


0-0717 


1-58 


1-287 


0-539 


748 


I-I74 


-0-495 


6 


0-0564 


1-58 


1-248 


0-414 


o-86i 


I-159 


-0-731 


7 


0-0409 


1-50 


1-225 


0-420 


0-805 


I -164 


—0-690 


8 


0-0262 


1-50 


1-179 


0-288 


0-891 


I -140 


— 0-629 


9 


o-oioo 


1-57 


i-ii8 


0-259 


0-839 


I-I02 




10 


0-0021 


1-62 


1-095 


0-209 


0-886 


I-09I 


-0-735 


II 


0-0002 


3-16 


0-728 




0-727 


0-727 


-0-516 


12 




15-78 


0-109 1 




0-109 


0-109 


+ 0-180 




Salt in original 


solutions 3-02 mols. per gram. 




VIII. I 


0-1817 


13-62 


3-495 


2-673 


0-822 


1-020 




2 


0-1465 


12-98 


3-028 


1-977 


1-051 


I-I27 






3 


0-1154 


12-57 


2-553 


1-558 


0-995 


I-I02 






4 


o-o86i 


11-63 


2-088 


1-093 


0-995 


1-087 






5 


0-0722 


10-93 


1-842 


0-855 


0-987 


1-052 






6 


0-0570 


11-73 


1-698 


0-871 


0-827 


1-029 






7 


0-0347 


11-52 


1-426 


0-508 


0-918 


1-026 






8 


0-0247 


10-99 


1-295 


0-465 


0-830 


1-024 






9 


0-0096 


10-73 


1-142 


0-221 


0-921 


1-039 






10 


0-0023 


9-81 


I-OIO 


0-147 


0-863 


0-987 


. 




II 


0-0002 


10-44 


0-684 




0-684 


0-682 






12 




11-90 


0-103 




0-103 


0-103 







Salt in original solutions 0-76 mol. per gram. 
This acidity is original acidity of gelatine, due principally to bisulphites. 



6o4 PRINCIPLES OF LEATHER MANUFACTURE 

imply that the acid actually attracted by the gelatine is greater 
when salt is present, but that the dissociation or hydrolysation 
is less, or, on the other hand, that the obviously doubtful assump- 
tion that the concentration in acid of the absorbed solution is 
equal to that of the external is incorrect. If, as is probable, the 
concentration of the absorbed solution is really less, the apparent 
" fixed " acid will increase in some inverse ratio to the volume of 
the solution absorbed. 

On the other hand, it must be noticed that the less ionised 




Fig. 124 (Table XII.). — Abscissae; Mgr.-moleculesNaClin i grm. solution. 
Ordinates ; x =Mgr.-inol. HCl in i grm. dry gelatine ; 0= concentration of 
acid fixed by i grm. dry gelatine ; + =wt. of solution absorbed by i grm. 
dry gelatine ; O =" fixed " salt. 

acid, as determined by its non-effect on the methyl orange, is 
proportionately, and even actually, less in the salted than in the 
merely acid solutions. It is, however, by no means probable 
that methyl orange is whoUy insensitive to the acidity of the 
acid-gelatine, and, unless this is the case, the effect of the latter 
will be increased by its greater concentration in the contracted 
jelly, just as methyl orange is reddened by many organic acids 
in concentrated solution which scarcely affect it when dilute. 
Any direct action of the acid-gelatine on methyl orange will 
diminish its apparent " fixed acid " as determined by this means. 
An interesting point in Tables XII. and XIII. is that of the 
amount of salt " fixed," which in presence of hydrochloric acid 



PAPERS ON GELATINE EQUILIBRIUM 605 

appears to be always negative, the few apparent exceptions being 
obviously due to experimental errors, and accompanied by ab- 
normal figures in one or other of the remaining determinations ; 
while Table XI. shows that in absence of acid, a varying positive 
amount is " fixed." The quantities were calculated by deter- 
mining the total chlorine in the treated gelatine with argentic 
nitrate and potassium chromate, and subtracting from this 
the hydrochloric acid found acidimetrically, and the total chlorine 
calculated in the solution absorbed. From the large total amount 
of chlorides present, and the proportionately large effect of the 



10 



■05 



•15 



r . 



Fig. 125 (Table XIII., series VII.). — Abscissae ; Mgr.-mol. HCl in i grm. 
solution. Ordinates ; • =Mgr.-mol. HCl in i grm. dry gelatine ; + =wt. of 
solutionabsorbedby I grm. dry gelatine (&) ; 0=" fixed" acid; ;ir=" fixed" 
salt. 

absorbed solution, great quantitative accuracy cannot be claimed 
for these figures, but they strongly support the view that the 
chlorine absorbed as hydrochloric acid by the gelatine exerts a 
corresponding expulsive effect on the chlorine ions (both acid 
and salt) contained in the solution. It is noted in connection 
with the experimental work that in several cases where large 
quantities of salt were used the sheets of gelatine, instead of 
appearing homogeneous and transparent, became white and 
opaque ; and this must have been due either to the actual crystal- 
lisation of salt in the gelatine, or to the formation of a cellular, or 
at least heterogeneous structure enclosing solution of a different 
refractive index to that of the surrounding jelly. It is extremely 
probable that this circumstance may account for the irregularity 
of the figures with regard to fixation of salt. 

From the curves given in fig. 124 it will be seen that the swelling 



6o6 PRINCIPLES OF LEATHER MANUFACTURE 

diminishes in a curve of hyperbolic character with increasing 
concentration of salt, becoming as5/mptotic to a value apparently 
somewhat above zero. 

Apparently any acid and its corresponding neutral salt will 
produce a dehydrating effect on swollen gelatine, varjdng in 
intensity with the electrolytic dissociation-constant of the acid, 
and increasing with the concentration of the salt. The effect of 
sodium sulphate with sulphuric acid appears to be quite as great 
as that of sodium chloride and hydrochloric acid in equivalent 
concentrations, while that of the weak acids with their own 
neutral salts is much less, though distinctly marked when com- 
pared to the swelling action of the acids alone, as will be 
obvious in comparing Tables XV. and XVI. with VI. and VIII. 
With the weaker acids, the excess of acid absorbed in excess of 
that due to absorbed solution (" apparent fixed acid ") is much 
less and very irregular as compared to that observed with the 
stronger acids ; and this is particularly the case with the feebly 
ionised acetic acid, of which the ionisation is still further reduced 
by the presence of its neutral salt. 

It is not necessary in order to produce contraction of swell- 
ing that the salt employed should have the same anion as the 
acid, and if the salt is one of a strong acid and in sufficient con- 
centration, good contraction may be obtained even by acidifica- 
tion with very weak acids. Thus effective pickling may be 
obtained with formic or acetic acid in presence of common salt ; 
and at the writer's suggestion formic acid has been to some extent 
used commercially for sheep-skins, as the antiseptic effect is 
even greater than that of the customary sulphuric acid, while 
certain injurious effects of the latter on the final manufactured 
product are avoided. In the ordinary commercial process, as 
has been stated, sulphuric acid and common salt are employed ; 
but except for questions of cost, hydrochloric acid and sodium 
sulphate would be equally effective, as shown in Table XVII. 
Table XVIII. shows results with acetic acid and common salt. 
It will be observed that both the dehydration and the acid 
" fixed " are practically the same with acetic acid as they would 
have been if hydrochloric acid had been used. 

Table XIX. gives a series in which potassium chloride is sub- 
stituted for sodium chloride with hydrochloric acid. The results 
are again practically identical with those of sodium chloride. 



PAPERS ON GELATINE EQUILIBRIUM 607 



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PAPERS ON GELATINE EQUILIBRIUM 609 



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6io PRINCIPLES OF LEATHER MANUFACTURE 



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PAPERS ON GELATINE EQUILIBRIUM 6ii 



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PAPERS ON GELATINE EQUILIBRIUM 613 

Summary 

Gelatine absorbs water with evolution of heat, and is capable 
of exerting large external pressures in the initial stages, but as 
the quantity of water becomes greater the avidity of the gelatine 
becomes less, and there is no further perceptible evolution of heat, 
while the mechanical force exerted is exceedingly slight, and may 
be measured in a few dynes per square centimeter. The process is 
completely reversible, and water may easily be expelled from the 
fully swollen jelly by mechanical pressure, and completely re- 
moved by evaporation in vacuo, and to a large extent by dehy- 
drating agents, though as dryness is approached the last portions 
of water are removed with great difficulty. The swelling does 
not proceed to infinity in cold water^ but reaches a definite 
maximum, which is an equilibrium between the affinity for water 
and the elastic cohesive forces of the gelatine, which are influenced 
not only by its chemical character but by its original volume at 
setting. In hot water complete solution takes place. 

Gelatine is insoluble in, and water-swollen gelatine is imper- 
meable to, strong alcohol, which dehydrates and compresses it. 
If, however, alcohol is incorporated in the liquid jelly, which is 
subsequently allowed to set, and immersed in water, the swelling 
is increased beyond the ordinary maximum. 

Gelatine swells in very dilute acids to a much larger extent 
than in water. In weak acids this swelling increases with the 
concentration of acid till finally solution takes place, but with 
strong acids the swelling reaches a maximum at very low dilutions, 
and subsequently diminishes in a curve of hyperbolic character 
till the jelly dissolves without further swelling, which limits the 
possible range of experiment. If, however, a neutral salt of the 
acid be added, the dehydration may be carried with strong acids 
to a point at which the gelatine forms a solid and horny mass. 
With weak acids and their salts the effect is less marked, but quite 
obvious. If, however, a weak acid is used with large excess of 
a salt of a strong acid the gelatine behaves as if the strong acid 
only was present, while gelatine swollen with a strong acid and 
treated with the salt of a weak one naturally behaves as if the 
weak acid had been used. 

In all cases the acid absorbed by the swelling gelatine is in 
excess of that due to the absorbed solution, and this excess is 
in any one series of experiments of an approximately constant 
amount over a wide range of concentration of the acid or acid 
and salt solution ; while under no conditions does it pass a 
maximum of about 1-25 mgr.-mols. of acid per gram of dry 



6i4 PRINCIPLES OF LEATHER MANUFACTURE 

gelatine. In the case of strong acids which affect the colour of 
methyl orange even at very great dilutions, it is shown that a 
portion of the acid absorbed by gelatine, which varies with the 
conditions of the experiment but always falls within the limit 
above stated, has become so considerably less ionised than the 
free acid that it is incapable of affecting the colour of methyl 
orange, though it may still be estimated by phenolphthalein. 

In the case of common salt it is shown that while in neutral 
solutions it increases the swelling of gelatine, and an amount is 
absorbed in excess of that normally contained in the absorbed 
solution, in presence of even small quantities of hydrochloric 
acid great dehydration is produced, and the fixation of salt is 
negative. 

Theoretical 

As regards the explanation of the foregoing experimental 
results, anything which can yet be said must,' in view of the pre- 
liminary character of the investigation and the somewhat rough 
methods of experiments, be regarded as merely working hypo- 
thesis. As a preliminary to this it is necessary to have some 
definite conception of the actual structure of a gelatine jelly, and 
the view which is here adopted is that of a network of gelatine 
molecules cohering to each other, but leaving interstices of 
molecular dimensions containing water or aqueous solutions, 
which, being within the range of molecular attractions, are really 
semi-solid solutions in the gelatine, and have with it a common 
internal pressure. The gelatine molecule, consisting as it does 
of a complicated chain of amido-acids, is peculiarly fitted to pro- 
duce such a structure. The range of molecular attraction does 
not apparently exceed lo /xju, (millionths of a millimeter), and 
may be much smaller {cp. Freundlich, Kapillarchemie, S. 277) ; 
but as about 2 per cent, of gelatine is required to form a coherent 
jelly, there would be in each cubic space of (10 /xju,^) a weight 
of gelatine over thirteen million times the estimated weight of a 
hydrogen molecule, and therefore ample molecules for a net of 
molecular dimensions. The facts mentioned as regards the effect 
of concentration at the moment of setting on the subsequent 
swelling give considerable support to the idea of a molecular 
network formed at the time. 

This view, though apparently very similar to the currently 
accepted one of van Bemmelen and Biitschli, is really very 
different, since these investigators assume a cellular structure of 
microscopic dimensions, and hence far beyond the range of mole- 
cular forces. That such cellular jellies or pseudo- jellies exist and 



PAPERS ON GELATINE EQUILIBRIUM 615- 

can be produced is undeniable, but in the writer's, opinion it is 
quite unproved that any such structure naturally exists in aqueous 
gelatine jellies, the microscopic observations all having been 
made on jellies hardened and shrunk with dehydrating agents, 
while no structure could be detected in the unhardened jelly, in 
which its dimensions, if existent, must have been much larger. 
A dilute gelatine solution which has been sufficiently heated for 
complete solution shows only stray, and probable accidental, sub- 
microns, and merely a shght Tyndall effect in the ultramicroscope, 
so that it is probably, for the most part, a true molecular solution 
in which the gelatine is uniformly distributed throughout the 
liquid. On cooling no fiocculation or visible contraction of the 
gelatine takes place, but the whole solidifies to a transparent and 
apparently homogeneous jelly, and it is hard to imagine how, 
under these conditions, a cellular structure could be formed. In 
some other cases, as, for instance, in the fiocculation of albumin 
solutions by heat, where the gel is obviously less hydrophile than 
the sol, the network, even if at first uniform and molecular, 
necessarily contracts, and a reticular structure must result ; while 
when a solution separates into two partially immiscible liquids 
an emulsion will be formed, which may be a pseudo-jelly ; but 
none of these causes appear to exist with gelatine, and the assump- 
tion of a cellular structure does not in any way assist, but con- 
siderably complicates the explanation of observed facts. .. 

It is clear that if we accept the conception of a jelly as the 
solution of a liquid in an elastic solid the whole question of 
swelling becomes one of osmotic pressures or, perhaps more 
accurately, one of distribution between two immiscible solvents. 
In the latter case the common surface takes the place of a semi- 
permeable membrane, and as the solvents are no longer identical, 
the partition-constant {Teilungscoefficient) , and sometimes also 
a different molecular complexity, have to be considered. There 
is also an important difference between the two cases which may 
be overlooked. In ordinary osmosis the molecular attractions of 
the common solvent merely serve to overcome and balance those 
of the solute and bring it into a region of equal and common 
internal pressure, where its kinetic energy can exert itself against 
a fixed semi-permeable septum, so that in the terms of gas equation 
V is constant and P varies. In the equilibrium of immiscible 
solvents, on the other hand, the kinetic energy of the solute is 
constant, and it is the varying internal attraction of the two 
solvents for its molecules which determines the partition-con- 
stant ; and since the diffusion-surface is freely movable, P is 
constant and V varies. This latter statement is somewhat modi- 



6i6 PRINCIPLES OF LEATHER MANUFACTURE 

fied in the case of jellies by the residual forces of solid cohesion 
which oppose change of volume. It can be shown, however, that 
in or near the region of maximum swelling these forces are 
extremely small. P. von Schroeder ^ showed that a jelly swollen 
in water lost a large amount of weight and volume (up to 73 per 
cent.) in an atmosphere of saturated water-vapour, but that this 
was completely prevented if the jelly were saturated with a 
N io~^ solution of an alkahne sulphate, and considerably 
lessened by one of N io~^. It was subsequently shown by the 
present writer ^ that the energy involved in removing i grm. 
water against the osmotic pressure of a N io~^ sulphate solution 
(about 350 ergs) was quite comparable to that done against 
surface tension in forming the surface of a sphere i grm. weight, 
and that no doubt the shrinkage might be accounted for in that 
way. It therefore follows that in most cases of considerable 
swelling the solid cohesion is almost negligible as compared to 
osmotic forces. On the other hand, in the earlier stages of absorp- 
tion of water by dry organic colloids, including gelatine, the 
forces involved are large, as is evidenced by the marked evolution 
of heat, the contraction of common volume, and the very con- 
siderable pressures obtained when the swelling is opposed by 
mechanical obstacles. 

If the swelling of gelatine in pure water be admitted to be 
osmotic, still more evidently is this the case with regard to the 
dehydrating action of alcohol on the swollen jelly, since the latter, 
though freely permeable to water, is practically impermeable to 
alcohol, and the jelly-mass acts as a simple osmotic cell. The 
curve of swelling in mixtures of water and alcohol (Table II.) is 
of a simple type, corresponding closely to a rectangular hyperbola 
in the middle portion but diverging at both extremes, possibly 
because of the solid rigidity of the jelly. There is no reason to 
doubt complete reversibility. Alcohol shows considerable osmotic 
pressure in an osmometer with gelatine membrane, and it is shown 
that alcohol incorporated in jelly must produce a cellular struc- 
ture, in which the jelly acts as a semi-permeable membrane. It 
is possible that more complete investigation of the dehydrating 
effects of alcohol on jelhes might afford some definite information 
on their cohesion and osmotic pressures, since the action is purel}/ 
physical. 

The explanation of acid swelhng and the pecuhar maximum of 

its curve, and of dehydration of acid gelatine by neutral salts, is 

much more complex, and involves chemical as well as purely 

osmotic considerations. In what follows, the most detailed con- 

^ Collegium, 1903, p. 204. ^ Brit. Assoc. Rep., 1908, p. 216. 



PAPERS ON GELATINE EQUILIBRIUM 617 

sideration will be given to the action of hydrochloric acid and 
sodium chloride, as these have been most fully investigated, and 
there is no reason to think that the results differ in principle 
from those of other acids and their corresponding salts. 

Gelatine jelly is known to be very permeable to both acids and 
salts and to their ions, so that it is not easy to see how either 
can exert a direct osmotic pressure on the jelly-mass. It has, 
however, been shown that even very dilute hydrochloric acid is 
absorbed with some avidity by the jelly, which always contains 
acid in considerable excess of an equal volume of the surrounding 
solution with which it is in equilibrium ; and this excess, rising 
rapidly at first, soon becomes an almost constant quantity (see 
curve of " fixed acid " in fig. 123), strongly suggesting the idea 
of a definite though hydrolysing chemical compound of the nature 
of a salt, in which the amphoteric gelatine acts as base. This 
idea is further supported by the fact that an approximately corre- 
sponding quantity of acid becomes neutral to methyl orange, 
though it can still be hydrolysed by sufficient excess of water, and 
estimated by titration with caustic alkali with phenolphthalein 
as indicator. If such a gelatine chloride exists, it is extremely 
probable that it will be much less permeable for hydrochloric 
acid and other chlorides than the neutral gelatine, or, if we regard 
the jelly as a solution, that the solubility of salts of a common 
anion in the acid jelly will be much less than in the neutral, 
while its affinity for water is likely to be greater, owing per- 
haps to its greater ionisation. The jelly, of course, must be in 
equihbrium with the surrounding solution in every respect, and 
firstly in regard to hydrolysis, in which it will obey the ordinary 
law of mass-action. As regards its volume, the case is quite 
analogoiis to that of solutions of an acid and its salt separated 
by a movable septum permeable to the acid and to water, but not 
to the salt, which in the present instance is an indiffusible colloid 
jelly. The two solutions must be in complete equilibrium, and 
as the salt cannot diffuse, water and acid must pass through the 
septum till equilibrium is reached. Firstly, then, the common 
anion must be at equal concentration in both solutions, and 
because of the salt anions, the free acid must necessarily be less 
concentrated in the salt solution than in that of the pure acid. 
Secondly, the anion of the acid in salt solution must be in equili- 
brium with that of the salt itself, and this can only occur by 
absorption or expulsion of water or acid till the ionic pressure of 
the salt is equal to that of the acid contained in its solution. 
Thus, the necessary data being given, the volume of the salt 
solution or jelly is definitely fixed, and dependent on the con- 



6i8 PRINCIPLES OF LEATHER MANUFACTURE 

centration of the acid ions and the ionisation and quantity of 
the salt. Again, the quantity of unhydrolysed and ionisable salt 
for a fixed quantity of base depends on the hydrion concentra- 
tion of the acid and the hydrolysis constant of the salt, and thus 
different acids with different constants may produce very varied 
effects of swelling. 

One consequence of what has just been said must be remarked. 
It is evident that the " fixed acid," calculated on the assumption 
that the absorbed solution is of equal acid concentration to the 
surrounding acid, does not represent the whole of the unhydro- 
lysed gelatine salt, but is less than it by the amount of acid ex- 
pelled by the salt from its solution. Whether the whole of this 
acid is expelled by the ionised salt, or a part also by the un-ionised, 
which demands water for its solution, is not easy to determine, but 
probably unimportant for the general theory of the equilibrium. 

It is of course impossible to treat the problem at present in any 
rigid mathematical way while so manv of the factors are un- 
known, and especially that of the cohesive elasticity of the jelly, 
but it may be interesting to see how far the experimental results 
agree with theoretical assumptions. 

We have assumed that the combination of gelatine with hydro- 
chloric acid is of the nature of a salt of a weak base with a strong 
acid, of which the hydrolysis, according to Wm. Ostwald, is repre- 
sented by the equation — =—=k, when h is the hydrion con- 

h Kg 

centration, h that of (colloid gelatine) ions, and B of the hydro- 

lysed and unionised base (gelatine). K^^ is the dissociation 

constant of water, K^ that of gelatine as a base. It may be taken 

without serious error that at the dilutions in question HCl is 

fuUy ionised, and that therefore its concentration x in the solution 

is proportional to h, the concentration of the hydrions. h is the 

concentration of the kation of the ionised but unhydrolysed salt, 

and as the quantity of gelatine is constant in the experiment, B 

is obviously equal to i — &. The equation therefore takes the form 

(i — h)x X 

— ; =k, and resolving this as regards h, we have &= r as 

the measure of the proportion of unhydrolysed salt.^ The same 

expression may also be obtained from the ordinary dilution 

a"- ■ . 

equation r-=k, which, substituting the hydrolytic for 

^ {i—a)v 

the dissociation constant, also applies to moderate hydrolysis. 

k 
1 The hydrolysed portion is obviously • 



PAPERS ON GELATINE EQUILIBRIUM 619 

In this formula a is the hydrolysed portion, which above has 
been called 1 — 6, and i — ais the unhydrolysed salt b, while the 
concentration of the hydrolysed acid is the same as x, that of 
the external solution. Substituting these values and resolving as 

X 

regards h, we again get h= -. It will be observed that v 

X ~r /? 

does not appear in this expression, since the concentration is 
determined by x, and therefore hydrolysis is not affected by the 
volume of the jelly. The value of b obviously rises rapidly at 
first and tends to unit value, and the rise is the more rapid and 
the later part of the curve the more horizontal the smaller the 
value of k. It must be observed that Ostwald's equation is 
merely an approximative one, and specially uncertain as complete 
hydrolysis is approached. 

The unit value to which the expression tends is to that of a 
molecule or equivalent of the salt. In the actual experiments i 
grm. of gelatine was used, and neither its molecular weight nor 
its valency is known, and it is quite possible that the latter may 
vary, since as the acid becomes more concentrated it probably 
attacks additional amino groups. ^ It is impossible to calculate 
equivalent weights from the direct experiments with hydrochloric 
acid, since much free acid solution is absorbed in swelling, and it 
has been shown above that this is not of the same concentration 
as the external solution. In very dilute external acid the error 
is not considerable, since the total acid of the solution absorbed 
is small compared to that fixed by the gelatine, but in this case 
the hydrolysis is also very large. It has been shown, however, 
that even with the highest concentrations of acid used the swell- 
ing can be reduced to very small proportions by the addition of a 
sufficient excess of a salt with a common anion {e.g. sodium 
chloride), and as the anion does not increase but depresses the 
hydrolysis by lessening the ionisation of the salt, we may safely 
assume that the hydrolysis in this case is almost nil. Under 
these circumstances we find that the total acid absorbed by i 
grm. of gelatine is about 1-3 mgr.-mols. while that in the absorbed 
solution does not at most exceed 0-05 mgr.-mols., so that we may 
roughly assume that under the conditions 1000 mgr. of gelatine 

1 It may be noted that both Tables III. and IV. show a sudden and rather 
considerable rise both in the total acid and in the " fixed " acid (as deter- 
mined by calculation from the volume of the solution absorbed) at the 
concentration above 0-2 N ; and though this may be merely due to ex- 
perimental error, it is more probable that it is an indication that at about 
this concentration a further amino-groiip of the gelatine is attacked, 
corresponding to its increased solubility in the acid. 



620 PRINCIPLES OF LEATHER MANUFACTURE 

combine with 1-28 mgr.-mols. of acid, or, in other words, that the 
equivalent (though not necessarily the true molecular) weight 
of the gelatine, considered as monovalent, is about 780. It will 
therefore be necessary to multiply the values of x by 1-28 to 
obtain the actual acid combined with the gelatine base. 

We may now consider the further question of how the volume 
of swelling is connected with the absorption of acid. The ex- 
perimental curve is a very peculiar one with a marked maximum, 
indicating that at first the swelling increases rapidly with in- 
creased concentration, but afterwards diminishes inversely in a 
slower ratio. According to the theory which has been suggested, 
the swelling is due to the superior solubility or attraction for 
water of the ionising gelatine chloride as compared to the non- 
ionising neutral gelatine, and it should therefore increase directly 
with the increase of the chloride. On the other hand it is re- 
pressed by the anion of the acid solution, which not merely 
causes the concentration of the jelly with expulsion of water till 
its total ionic pressure is equal to that of the acid, but also 
compels the expulsion of free acid from the jelly against the 
same pressure of x, so that the total anion-pressure of the jelly- 
mass may remain equal to x. The force required to compress 
the jelly therefore varies not merely as x but as x^, and for a 
given increment of x the swelling should diminish as Vx. We 
should expect therefore that swelling would be represented by 

such an expression as [3 '-Vx, or ^ , where x and k 

x+k x+k 

have the same values as before, and [3 is an empirical constant 
connecting the volume of the jelly with its pressure. Such an 
expression will obviously give a marked maximum for some small 
value of x, and by differentiation it is shown that this maximum 
occurs when x=k. We have thus a means of arriving at a 
value for k, since the experimental maximum is clearly marked 
at about a;= 0-005 N, and the swelling curve on fig. 123 is there- 

Vx 
fore calculated as 7-8 . It is possible that the true value 

x + k 

of k is somewhat less than 0-005 N, and that the higher maxi- 
mum which this would cause is prevented by the cohesion of the 
jelly ; and since the curve rises much more abruptly than it 
falls, the effect of this rounding off of its summit would be to shift 
the apparent maximum to a slightly higher concentration ; but 
in any case, if the hypothesis be correct, it cannot be much lower 
that 0-005, 3-n^cl at least the order of the quantity remains 
unchanged. 



PAPERS ON GELATINE EQUILIBRIUM 621 

Since k=—, and K^, is known, we can calculate an approxi- 

mate value for Kg, the ionisation constant of neutral gelatine, 

o-G + io"-"-* 

as r, or of the order of i x lo"^^. It would be interest- 

o-5 + io~2 

ing to confirm this by more direct measurement. 

Of the direct experimental results, only the value of the total 
absorbed acid remains to be calculated. This is evidently (from 
the mode of calculation of the " fixed acid ") the sum of the 
latter and of the volume of absorbed liquid in the jelly multi- 
plied by the concentration of the external solution. As the 
former approximates closely to o-8 mols. from the maximum of 

swelling onwards, and the latter to x- , the curve has been 

x + k 

plotted on the sum of these, and sufficiently well expresses the 
experimental values, but offers no explanation of the amount 
and constancy of the fixed acid — a question which is discussed 
in the next paragraph. It is, however, interesting to note that 
the curve of absorbed acid is also well represented by y=8y x^-'^^, 
the logarithmic plotting of the smoothed curve being only slightly 
convex to the origin, and showing that curves quite of the adsorp- 
tion type may arise as the sum of purely chemical actions. 

It has been assumed that the resistance of the jelly to com- 
pression by the acid of the outer solution is the product of the 
direct ionic pressure of the gelatine salt, and of that of the anion 
of the acid which is expelled along with the water to maintain 
equilibrium, but which must carry with it its corresponding 
hydrion. The swelling pressure within the jelly is thus the sum 
of three partial pressures, of which only one is that of the anion 
itself, and the acid expelled is therefore only one-third of the 
total ionised gelatine salt. Since between x=o-^ and x=o-^ 
the quantity of gelatine chloride and its ionisation remain sensibly 
total and constant, and what change takes place in the one is 
partially compensated by the corresponding change in the other, 
the " apparent fixed " and the expelled acid will also remain 
constant, the one being about o-8 and the other about 0-4 mols. 
We have as yet no definite information as to the ionisation of 
the gelatine salt, the repression of which is no doubt negligible 
within the limits named, but must become considerable in the 
presence of much sodium chloride or other salt with a common 
anion. In this case the acid fixed as un-ionised gelatine salt will 
'increase and that expelled will diminish, so that the curve of 
fixed acid must more and more closely approach that of the 



622 PRINCIPLES OF LEATHER MANUFACTURE 

un-ionised gelatine salt, as under these circumstances it is experi- 
mentally shown to do. 

A point much more difficult of explanation is the almost vertical 
rise of the fixed acid curve near the origin, and its slight maxi- 
mum corresponding to that of the swelling curve. It is obvious 
that the reaUy combined acid cannot exceed that of a correct 
hydrolysis curve, and no constants or modifications can be 
adopted for the latter to make its rise more rapid which do not 
at the same time throw the swelling curve derived from it entirely 
out of harmony with experiment. It is perhaps a somewhat 
forced, though not, I think, an altogether improbable, explanation, 
that the uncombined gelatine at first adsorbs acid without actual 
combination, and that this adsorption is favoured by the large 
volume of the jelly at the point of maximum swelling, where the 
small maximum also occurs on the fixed acid curve. This adsorp- 
tion would of course tend to increase the apparent fixation of 
acid and render it more uniform, and would disappear with the 
disappearance of free base, since it is improbable that the chloride 
would adsorb hydrochloric acid. Some alternative explanations 
may be suggested, but it is better to wait the results of further 
experiment.^ 

It must, I think, be admitted that the complicated system of 
curves which has been deduced from the theory of actual chemical 
combination shows an agreement with experimental results 
which is more than accidental, and which in some sense really 
represents facts, whether these be strictly chemical or not. • It 
is not to be denied that an equally plausible hypothesis might 
conceivably be based on physical adsorption, but to be satisfactory 
it must show a correlation of experimental facts at least as com- 
plete as that which has been offered. It is to be admitted that 
much remains to be done in explaining the still outstanding 
deviations, in adducing further proof, and in replacing empirical 
constants by those based on definite knowledge, but the present 
paper at least provides a working hypothesis. The most marked 
deviation of fact from theory consists in the more rapid rise of the 
" fixed acid " to a small maximum above that allowed by the 
curve of unhydrolysed salt, and disregarding probable imper- 
fections in the dilution equation in such an extreme case, it is 
suggested that this may be explained by actual adsorption pre- 
liminary to chemical combination. 

It is obvious that the theory affords a complete qualitative 
explanation of the dehydrating eftect of common salt solutions 

1 The explanation is completely given by the theory as subsequently 
developed. See. Trans. Chem. Soc, 105, 1914, P- 325. (H. R. P.) 



PAPERS ON GELATINE EQUILIBRIUM 623 

and other chlorides on gelatine (including skin and connective 
tissue) swollen with hydrochloric acid, although our knowledge 
of the laws of concentrated solution is insufficient to enable us 
to apply the same equations quantitatively. The theory asserts 
that the falling of the swelling is due to the pressure of the 
chlorine ions, and it is indifferent whether these are furnished 
by the acid or some other chloride. That salt not only exerts 
a compressing pressure on the chloride jelly, but is expelled by 
the latter from the absorbed solution is proved by the last column 
in Tables XII. and XIII., in which the fixation of salt is shown to 
be negative ; though in presence of so much salt, the chlorine 
estimations cannot claim the same degree of accuracy as the 
acid ones. That salt exercises no compressing but rather a 
swelling influence on neutral gelatines is shown by Table XL, in 
which also the salt absorption is positive. The effect of salt in 
raising the apparent acid " fixed " is also easy of explanation, 
since the presence of the chlorine ion limits hydrolysis by repress- 
ing ionisation and increasing un-iohised chloride, and prevents 
expulsion of acid from the absorbed solution by being itself 
expelled in its place. A question arises, however, as to whether 
the compression of the jelly under these circumstances should be 
treated merely as a case of ionic equilibrium, or not rather as a 
" salting out," in which the avidity for water of the different 
constituents is more important than their ionic pressure, and in 
which the un-ionised as well as the ionised sodium chloride plays 
its part. Throughout, it has been assumed in the equations that 
the whole of the gelatine chloride, and not merely its ionised 
or un-ionised part, is effective in the swelling ; but obviously, if the 
ionisation is comparable to that of most salts, it must practi- 
cally total up to concentrations such as were used in the acid 
experiments. 

It remains, however, to be considered whether the theory 
developed from the study of hydrochloric acid is one of general 
applicability to all acid swelling of gelatine or gelatinous tissues, 
or whether it is merely an exceptional case confined perhaps to a 
few of the stronger acids, since sulphuric acid and sulphates show 
completely parallel effects (see Table IX.). 

It has been often stated that since a swelling effect was common 
to all acids of sufficient hydrion concentration, the hydrion must 
be regarded as the active swelling agent ; but this in the light 
of the present theory must be regarded as only indirectly the 
case, since the hydrion concentration is the measure of the 
avidity, and hence of the salt-forming avidity of the acid ; and it 
is quite possible that, as in the case of armnaonium, salts and many 



624 PRINCIPLES OF LEATHER MANUFACTURE 

other organic bases, not only the anion but the hydrion enters 
into the salt. In all cases the anion is the compressing and de- 
hydrating agent ; and in the absence of a neutral salt, a maximum 
with subsequent contraction is produced by the acid itself. This 
is, however, only obvious in the case of the stronger acids, since 
the weak acid, although it may produce a gelatine salt, is so little 
ionised that its anion cannot reach a sufficient concentration to 
overcome the pressure of the more ionised gelatine salt. Thus, 
while sulphuric acid produces a marked maximum, that of formic 
acid, though still obvious, is much less distinct, and with acetic 
and lactic acids none is observable, and the swelling continues 
to increase with concentration, becoming in the end greater than 
that produced by the stronger acids, and going on without a 
break to the final solution of the gelatine. In these cases the 
apparent " fixed acid " is somewhat variable, sometimes higher and 
sometimes lower than that of strong acids according to whether 
the ionic concentration is sufficient to prevent hydrolysis or to 
attack additional amino groups. 

It may be objected that sodium or potassium chlorides will 
produce vigorous dehydration not only with hydrochloric acid, 
but with any other acid of sufficient hydrion concentration to 
produce swelling ; but a little reflection makes it obvious that any 
such combination simply leads to a quadruple equilibrium in which 
each acid is balanced against its own neutral salt. Thus in the 
case of acetic acid and sodium chloride we have gel-ions, acet-ions, 
and an enormous excess of sodium and chlorine ions, and if we 
imagine combination, we must by the law of mass-action 
have much gelatine chloride balanced against sodium chloride 
and little gelatine acetate against sodium acetate. Hence 
the rule, since the salt is always largely in excess, that with 
the salt of a strong acid the dehydrating effect is the same 
whether the acidification has been by a weak or a strong 
acid, and vice versa. Acetic or formic acid will produce . as 
effective a pickling with common salt as sulphuric or hydro- 
chloric acid. 

As regards other acids and their salts it has been shown that 
in all cases depression of swelling is caused by a sufficient addition 
of the neutral salt, but this is most marked with the salts of the 
stronger acids, possibly because with weak acids the ionisation 
of the salt added is insufficient to repress that of the gelatine 
compound itself. 

I do not propose in this paper to discuss the complicated effect 
of the action of salts on the swelling of neutral gelatine which 
has been specially investigated by Pauli, Hofmeister, von 



PAPERS ON GELATINE EQUILIBRIUM 625 

Schroeder and others/ but it may be suggested that it is quite 
conceivable that in many cases a portion of the salt undergoes 
dissociation or hydrolysis, and that both acid and base combine 
with the gelatine, and that the compound so formed swells or is 
compressed by the remaining salt solution according to its con- 
centration. Paessler ^ has shown that sodium acid sulphate is 
dissociated by hide-substance, only the neutral salt remaining 
in solution, and it is clearly proved in the case of chromic, ferric, 
and aluminic salts that a similar action takes place in which both 
acid and base (or a basic salt) are absorbed. This absorption of 
base may occur either by combination with opened-up COOH 
groups, or as complex salts such as are often formed by ammonia 
with other bases ; and some of these may form complex ions 
which are unrepressed by those of the simple salt in solution. 

The subject of alkaline swelling is also left untouched, but it is 
clear from some preliminary experiments that in certain respects 
it differs radically from acid swelling. For instance, swelling 
by sodium hydrate is not at all repressed by sodium chloride ^ but 
is so by higher concentrations of the hydrate, showing that in 
this case hydroxyl ions and not the kation Na are the swelling 
and repressing agents. It is hoped to pursue the question of 
alkaline swelling. 

It is obvious that the facts discussed in the paper have an 
important bearing on many physiological questions, and in this 
connection a crude attempt made in the early stages of the 
inquiry to imitate muscular contraction may be worthy of men- 
tion. A slender spiral platinum electrode was embedded in a 
cylinder of jelly which was immersed in a salt solution containing 
a second electrode. When the gelatine electrode was the anode 
of a sufficient current to electrolyse the salt the jelly contracted 
admirably, but its relaxation by a reverse current was somewhat 
unsatisfactory owing to the evolution of hydrogen, which broke 
up the gelatine. No doubt with a suitable depolariser better 
results might be attained, and it is quite possible that the con- 
densation of the anion by a mere surface potential difference might 
be sufficient to produce the effect. 

Pauli * has recently published a paper on albumen in which he 

^ Pauli (Paseheles), Pflug. Arch., 1898, 71, 336 u. 339. Pauli u. Rona, 
Beitr. z. Chem. Physiol, u. Pathol., 1902, 2, 25-26. Hofmeister, Arch, fur 
experim. Pathol, u. Pharmacol., 1888, 24, 424. Von Schroeder, Collegium, 
1902, p. 306. 

^ Wissenschaftliche Beilage des Ledermarkt, 1901, ii. 106. 

3 This has been shown by later work to be not strictly true, though 
the repression is very much less than in the acid equilibrium. (H. R. P.) 

* Zeitschrift fur Chem. u. Ind. der Kolloide, 1910, S. 241. 

40 



626 PRINCIPLES OF LEATHER MANUFACTURE 

supports very similar views to those just expressed, though 
laying more weight on the hydration of the colloid ion than has 
been done in the present work. In particular he strongly advocates 
the molecular as opposed to the cellular or network structure 
claimed by Biitschli. He also mentions the fact that acid jellies 
cannot be dehydrated by pure alcohol, which has been confirmed 
in the present investigation, though it has also been found that 
with acidified alcohol considerable contraction takes place. 



PART II.— THE ACID-GELATINE EQUILIBRIUM i 
By H. R. Procter and J. A. Wilson 

In an earlier paper by one of us {Transactions, 1914, 105, 313, 
Journal of the American Leather Chemists' Association, 1914, 
pp. 207-25) it was shown that gelatine forms hydrolysable 
salts with acids, that swelling is due to the ionisation of these 
and the osmotic pressure so produced, and that an equilibrium 
results, in which the concentrations of anion, hydrion, and 
ionised gelatine salt can all be expressed as functions of the 
concentration of acid in the external solution, within the limits 
of experimental error. In this earlier paper the concentration 
of the ionised gelatine chloride in the equilibrium gelatine-hydro- 
chloric acid was shown to be approximately Cl^== '\/o-02a; +0-0002, 
where Cl^ was the chlorine ionised from the gelatine salt, and x 
the concentration, in terms of normality, of the external hydro- 
chloric acid. (Through an error Clg was given as i/2;t; + o-02, 
but all actual calculations were made from the above formula.) 
It was also assumed that the numerical values in the expression 
were constants, as those adopted sufficiently closely represented 
the experimental results then quoted, but closer theoretical in- 
vestigation has shown that this is not strictly the case, but that 
both C\g and x are functions of a quantity e, which is the difference 
in osmotic pressure between two phases of which the ionic pro- 
ducts are equal, but in one of which the factors are unequal, 
and that it is this difference which causes the swelling of the jelly. 

In the paper cited, the hydrochloric acid and gelatine salt were, 
for the sake of simplicity, supposed to be wholly ionised, and as 
the ionisation in both cases is very high and the solutions were 
dilute, such an assumption was quite justifiable as regards experi- 
mental results. In the closer theoretical examination which we 
now propose, however, we must define the concentrations as 

^ Transactions of the Chemical Society, 1916, pp. 307 £f. 
626 



PAPERS ON GELATINE EQUILIBRIUM 627 

referring only to the actual ions ; and we shall again first con- 
sider the comparatively simple case of hydrochloric acid and 
gelatine, where the ionisation is in reality almost complete. 
The following system of notation will be employed : — 

At equilibrium : 

In the external solution : 

a;=[H+] = [C1']. 
In the jelly phase : 

^•^ concentration of gelatine ions. 

y+z=^[CV]. 

a = concentration of non-ionised gelatine chloride. 

^=sum of concentrations of gelatine, gelatine chloride, 

and gelatine ions. 
g=excess of concentration of diffusible ions of the jelly 

over that of the external solution. 
F= volume of the jelly in c.c. 

All concentrations are expressed in gram-equivalents per liter. 
In all experiments i grm. of dry gelatine was immersed in 100 c.c. 
of the acid solution and allowed to remain for forty-eight hours 
to reach equilibrium, the temperature being about 20° in each 
case. The concentration of acid in the external solution was 
determined by titration, and that in the jelly by titrating the 
solution expelled from the jelly by the addition of salt, and with 
these, knowing the initial concentration of the acid and the per- 
centage of ionisation of acid in the external solution, the actual 
ionic concentrations can be calculated. 

In a two-phased equilibritim, such as the present, two different 
equations must be fulfilled. It is necessary that the products of 
hydrion and chloridion should be equal in the two phases, that is 

x^=y{y+z) . . . . (i) 
This is not only proved by the thermo-dynamical equation of 
Donnan, quoted in the earlier paper {loc. cit.), but also follows 
from the ordinary laws of ionisation, since the non-ionised 
portion of hydrochloric acid which, although small, must exist, 
takes no direct part in the equilibrium, and must be equal in both 
phases since the jelly is permeable to it, and 

%2=[Hi+] X [C1\]=K[HC1]=[H,+] X [CV,]=y{y+z). 
It was, however, previously pointed out that the equations 

'x^-=y{y+z) 

2X = 2y+Z 



{; 



628 PRINCIPLES OF LEATHER MANUFACTURE 

cannot simultaneously be fulfilled, since in the jelly [H+] and 
[Cr] are necessarily unequal, the chlorion being greater than 
the hydrion by the amount z, and the sum of the sides of an un- 
equal rectangle in necessarily greater than that of the sides of 
a square of equal area (see fig. 126). In other words, 2y-\-zis 
greater than 2X by an amount we shall call e, and the corrected 
equation becomes 

2x + e=2y+z . . . • (2) 

The concentration of diffusible ions of the jelly, is therefore 
greater by e than that of the outer solution. It is obvious, since 
water and hydrogen chloride can pass freely into the jelly, that 
there must be some force equal to and opposing the osmotic 
pressure produced by this excess e of concentration at equili- 
brium, for otherwise the jelly would tend to swell to infinity. 

Before attempting to speculate about the nature of this oppos- 
ing force, we must consider its mathematical relations to the 
other concentrations of the equilibrium as defined by equations 
(i) and (2). The general theory of the equilibrium as developed 
in earlier papers (this book, p. 117, and J.A.L.C.A., 1914, 
pp. 207-25) is that when gelatine is placed in dilute acid it 
absorbs it freely and forms a hydrolysing salt, the propor- 
tion of which to the whole gelatine base present is determined 
by the hydrolysis equation. The gelatine salt, like other salts, 
is highly ionised into the anion and a colloid kation, which 
either from polymerisation or other causes peculiar to the 
colloid state cannot diffuse and exerts no measurable osmotic 
pressure, whilst its anion is retained in the jelly by the 
electro-chemical attraction of the colloid ion, but exerts osmotic 
pressure which, on the one hand, causes the mass to swell with 
absorption of the external solution, and, on the other, expels a 
portion of the acid, both anion and hydrion, from this solution 
absorbed, the result in equilibrium being that the jelly is poorer 
in hydrion and more concentrated in anion than the external acid 
solution, the difference of concentration between anion and 
hydrion in the jelly being, of course, equal to the ionised anion of 
the gelatine salt, and electrically balanced by the positive gelatine 
ions ; whilst the hydrion concentration in the jelly is less con- 
centrated than that of the outer solution by the amount of acid 
expelled, which may be called v (the isotonic volume of hydrion or 
chlorion expelled at a concentration oi x) ; v bears the simple 
relation to y that y + v=x, and the concentration of ionised 
gelatine chloride, z=2v + e. 

By solving simultaneously equations (i) and (2) the following 
interesting relations are derived : — 



PAPERS ON GELATINE EQUILIBRIUM 629 



x=y + Vey=Vy^ + 



yz'- 






y-- 



-z + \/z^+4x^ __2x + e-^4ex + e^ jz-e)^ 



4e 



x^—y^_ 

y 

^_{x-yY_ 

y 



V^ex- 



-e'==g- 



-2Vey 



z + 2y- 2 Vy2 ^yz= - 2x + V4;t2 +z^ 



These relations can be represented graphically for any value of 
X, as is shown in fig. 126. 



.13 




.12 








/ 


// 


.11 


ir 


on] 




/ 


// ,# 


10 


c- 


0A\ 




/ e/v / 


9? 


^y 


.09 
.08 

06- 


u 


lAj 


/ / 


/pyv^ 


Z 


f 








/ 




04 


^ 


/yy/ 


^ 


vr\\ 


: x=.io 
; i|=.o8 

:-^=.045 




/" 








; £=.005 


02 


^ 











.02 .04 .05 .08 .10 
Fig. 126. — Curves of Concentration. 

Any one variable can be derived in terms of any other two, but 
in no case, from only the two equations given, can an equation be 
derived containing only two variables. As was pointed out pre- 
viously, however, z was found from experiment to be equal 
approximately to Vo'02:*; + 0-0002, which bears a resemblance to 
one of the derived equations, namely, z=^/Afix + e^. Putting 
e=o-oo5 we get 2;= ^0-02^; +0-000025, which is strikingly 



630 PRINCIPLES OF LEATHER MANUFACTURE 

like the one obtained empirically, but gives low values for concen- 
trations less than ;v=o-03. Theoretical considerations rendered 
it improbable that e, considered in the general way in which 
we have done, could be a real constant, but it is difficult to obtain 

a smooth experimental curve from the formula e= , be- 
cause small errors in y correspond with large errors in e, and 
it is not possible to determine y in the most dilute solutions with 
very great accuracy by volumetric methods. 

It occurred to us, however, that if we could incorporate the 
volume of the jelly, which can be determined with great accuracy, 
into an equation containing only two other variables, it would at 
once be possible to calculate any variable from V and x only. 
It was found that the theory could be summed up in the form of 
such an equation, which could readily be subjected to the rigid 
test of experiment. The notation used is that mentioned earlier 
in this paper. The degree of ionisation of any given electrolyte, 
MN, is often expressed by the formula [M+] x[N']=i^[MN], 
where K may be nearly constant, as in the case of acetic acid, or a 
variable, as in the case of highly ionisable electrolytes. In the 
following, gelatine chloride is considered to be ionised into 
gelatine ion and chloridion, and the gelatine ion to be ionised 
still further into gelatine molecule and hydrion. A similar case 
would be the ionisation of ammonium chloride into ammonium, 
and chlorion and the further ionisation of the ammonion into 
ammonia and hydrion. We may therefore write : 

{a) [gelatine ion] x [CF] =i^[gelatine chloride] 

or z{y+z)—Ka. 

{b) [gelatine molecule] x [H+] =iir'[gelatine ion] 

or y{g — a— z) =K'z. 

By solving (a) and (b) simultaneously to remove the term in a : 

(c) Kgy=yh+Kyz +yz^+KK'z. 

Taking Procter's figure of 839 for the molecular weight of a 
unit of gelatine, i grm. of gelatine represents 0-00119 gram- 
equivalent. Therefore at any volume : 



{d) 



V 



Substituting {d) in (c) and simplifying : 



z(y''+Ky+yz+KK') 



PAPERS ON GELATINE EQUILIBRIUM 631 

Or in terms of x and y : 

i-ig Ky^ 

if) V=^ ^ 



{x^-y^) {x'-+Ky+KK') 

The value for K' has been considered to be so smah that 
neglecting it should produce no appreciable errors in concentra- 
tions greater than ;v=o-oo5, so that for these more concentrated 
solutions the equation reduced to : 

^^' {x^-y-^){x^+Ky) 

Now K has been assumed, with good reason, to be nearly equal 
to the ionisation-constant of hydrochloric acid for cori^esponding 
concentrations. It will be noted from the above equation that 
small errors in the value of K will produce negligible errors in the 
calculations so long as the value of Ky is considerably greater 
than that of x'^, and such a condition does obtain in these more 
concentrated solutions so long as K is the ionisation-constant of 
a strong electrolyte. For the present set of calculations, then, it 
will be permissible to take K as the ionisation-constant of hydro- 
chloric acid, which is known approximately for any given con- 
centration. Moreover, since it has been shown that, in these 
more concentrated solutions, almost all the gelatine has been con- 
verted into the monochloride, we are justified in taking K as the 
ionisation-constant of hydrochloric acid at concentration g, where 

g is simply — -. Now, from experimental values for V and x 

it is possible to calculate values for y, which should not differ 
from the value obtained from experiment by more than would 
be attributed to experimental error. The results are given in 
Table I. 

The agreement between experimental and calculated values 
bears out the theory remarkably well, the differences being not 
greater than was to be expected, considering the difQculties in 
titrating small quantities of solutions containing traces of organic 
matter. We feel that values for e calculated from V and x by 
this formula will be approximately correct. Of course it is 
evident that in the very dilute solutions K' ceases to be a negligible 
quantity. Prehminary experiments with the hydrogen electrode 
show that K' is of the order of 0-00015, which fully justifies our 
assumptions in neglecting it for the higher concentrations. 

In figs. 127 and 128 curves are given for the various variables as 
functions of x. The values of V, x, and y were obtained directly 
from experiment, whilst those of z and e were calculated by 



632 PRINCIPLES OF LEATHER MANUFACTURE 







Table I. 






Determined 










as noted 




By Experiment. 




Calculated. 


above. 1 




^. 






K 


^ V 


X 


y 


y 


0-95 


16-9 


0-262 


0-228 


0-237 


0-94 


17-5 


0-220 


0-186 


0-195 


0-88 


20-2 


0-174 


0-145 


0-152 


0-85 


21-6 


0-153 


0-123 


0-132 


0-85 


21-6 


0-130 


0-105 


0-108 


0-83 


22-4 


0-108 


0-080 


0-087 


o-8o 


24-1 


0-087 


0-066 


0-068 . 


0-75 


25-9 


0-064 


0-049 


0-047 


0-65 


34-3 


0-0386 


0-028 


0-026 


0-56 


45-6 


0-0165 


0-0084 


0-0084 


0-55 


49-4 


0-OII8 


0-0057 


0-0050 


0-50 


56-4 


0-0071 


0-0020 


0-0022 



means of the above formula. It will be noted by reference to 
figs. 127 and 128 that e apparently varies directly as the volume. 
Such a relation, if it could be proved, would simplify all other 
relations to the extent that any variable could be expressed, with 
suitable constants, in terms of any other one, and the theory, as 
such, might well be said to be complete. 

It was conceived that by means of the hydrogen electrode we 
could determine all the variables in a special experiment in which 
the volume was kept more nearly constant. One grm. of gelatine 
was dissolved in such a quantity of water that at 33° the volume 
was 21 c.c. This high temperature was used because it is neces- 
sary to make hydrion determinations in the melted jeUy. The 
method of using the hydrogen electrode was similar to that used 
and described by Sorensen. Acid was added at intervals and 
the hydrion concentration determined. From the figures 
obtained it was found possible to calculate all the variables 
desired. Knowing the quantity of acid added, it was possible 
' to calculate, from the hydrion concentration found, the amount 
which had combined with the gelatine, and consequently the 
total concentration of gelatine chloride, giving values for y and 
a+z. Whilst actually there is no external solution, there is a 
theoretical one, since the products of hydrion and chloridion 
must be equal in both phases ; that is, the product y{y+z) of 
the jelly is equal to the x^ oi the theoretical external solution. 
^ These calculations are based on the figures of Bray and Hunt [Journ. 
Amer. Chem. Soc, 1911, 33, 781) and those of Noyes and Falk {ibid., 34, 
454)- 



PAPERS ON GELATINE EQUILIBRIUM 633 

If, as before, we regard the gelatine chloride as ionised to about 
the same extent as hydrochloric acid, we can take K as the 
ionisation-constant of hydrochloric acid at a concentration of 





/ / / 




2 0.10 

t- 


2 






CO 

< 






00 
UJ 

_l 
CO 

<0.05 
^0,04 




— • — ' 


Q03 






002 






0.01 


vP^ ' ' 1 


e 







0.01 03 05 



0.10 
X 

Fig. 127. 



60 




55 


"1 


M 50 


■ L 


CJ 


\ 


Z 45 


\ ' 


>] 40 


\ 


d 35 


\^ 


~3 -,„ 


^s. 


Li- 30 


^^v^^ 





^'^■^'.....^^^^ 


UJ 25 


^'^"^"■■■-^-— _ 


2 




■=> 20 


^ 


_i 




^ i5 




10 




5 





0.01 0.03 0.05 0.07 0.09 

X 

Fig. 128. 



0.15 



0.20 



a-\-z, which is known. From the equation z[y-\-z)^Ka it 
follows that 

- {K +3;) + ^{K^yY^AK{a^z) 



in which all terms on the right-hand side are known, and conse- 



634 PRINCIPLES OF LEATHER MANUFACTURE 

quently z can be calculated. From y and z, also, all other vari- 
ables can be calculated by formulae presented earlier in this paper. 
The results obtained in this way are given in Table II , and are 
shown graphically in fig. 129. ^ 

The curve of special interest is that of the variable e, which is 
seen to increase to a maximum at a very low concentration and 
then to fall in a manner similar to that of the volume curve 



. 


.y^ 




/ 


0.064 


r\/ 


y 


^ 




y^ 


-K/ 




0.056 


/ 


/ 


V 


0.048 


X / 




/ 


UJ 

1 0.040 


_/ / 


/ 




/ ^-— " / 


' / 


~~*-*L. 


^ 


/X^'^^ / 


/ 




a 0.032 

< 


/ / y 






1 0.024 


1/—-^ / y 






0.016 








0.008 


\/^^ 


-— 


__,__e__ 



0.008 0.016 0.024 0.032 0.040 0.048 0.056 0.064 
X 

Fig. 129. 

(fig. 128). Since e represents the measure of an outward pressure, 
we have, when the jelly is free to swell, an application of a special 
case of Hooke's Law, ut tensio sic vis, where stress=5X strain ; 
and since g is a uniform pressure, it follows that it will produce 
an increase in the size of the 'jelly, but not in its shape, and that 
the increase in volume will be directly proportional to the pull. 
If we take the volume of i grm. of dry gelatine as 07 c.c, then, 
so long as the elastic limit is not exceeded, e=k{V—o-y), where 
the value of the constant is determined by the bulk modulus 
of the gelatine or particular protein under consideration. The 
relation is therefore dependent on the temperature, and that this 
is an appreciable factor is shown by the rough experiments 
tabulated on the following page. 

It is probable that the effect produced by this limited rise of 
temperature is not due to material changes in ionisation or 
chemical activity, bat almost solely to the diminution of the solid 



PAPERS ON GELATINE EQUILIBRIUM 635 

Volume of Jelly. 



Initial 




concentration 




of Acid. 


^ 7° 


0-200 


II-3 


o-ioo 


13-0 


0-050 


14-6 


0-025 


i8-5 


o-oio 


22-2 



15^ 33° 

17-6 

19-8 

23-2 33-0 ^ 

27-3 

34-3 

cohesion of the jelly. Many reasons convince us that the cohesive 
forces of the jelly opposing e are still maintained beyond the 
melting point. 

It is evident that the volume of the jelly, at a constant tem- 
perature, is dependent for its value on the value of e, and the 
only remaining question is why the value of e should follow a 
curve of the particular type that it does. As was noted earlier 
in this paper, the following equation results from a simultaneous 
solution of the thermo-dynamic and osmotic equations given : 



e=-2X + -\/^2^^2^ 

As the concentration of acid is increased from zero to some 
small, but finite, value, z must necessarily increase at a very 
much greater rate than x. This is shown very markedly in the 
most dilute solutions, where almost aU the acid added combines 
with the gelatine ; but z has a limiting value, which is determined 
by the total concentration of gelatine with which we started. 
Now z must either approach this limiting value or diminish, which 
it would do if the ionisation of the gelatine chloride were suffi- 
ciently repressed. In either case 

limit . , — 

^_oo V4x^+z^=V4x^=2x 

from which it follows that : 

^''^^e=~2X + 2X=0. 

It is clear from this that, as x increases from zero, e must increase 
to a maximum and then decrease, approaching zero asympto- 
tically, regardless of whether or not the ionisation of the gelatine 
salt is appreciably repressed. In fig. 129 it will be seen that e 
begins to decrease at a considerable rate while z is still increasing 
shghtly, v/hich would be expected. It should be noted that the 

1 The value for the volume of 33°, a temperature well above the melt- 
ing point of the jelly, is approximate only, and was obtained by gradually 
raising the temperature of the swollen jelly and its equilibrium acid, when 
the jelly, from its gravity and viscosity, does not mix with the supernatant 
liquid. 



636 PRINCIPLES OF LEATHER MANUFACTURE 



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PAPERS ON GELATINE EQUILIBRIUM 637 

apparent decrease in z in the most concentrated solution, given 
in Table II., is due chiefly to the increased volume. 

An interesting point is raised here regarding the action of salt 
in repressing the swelling of jelly swollen with acid. Whilst the 
salt undoubtedly represses the ionisation of the gelatine chloride 
to some extent, it would scarcely be sufficient to account for the 
fact that salt reduces the volume of jelly almost to that of dry 
gelatine. The chief action is probably that the addition of salt 
corresponds with an increase in the value of x, and that this 
increase in x must, according to the equation just discussed, pro- 
duce a decrease in the value of e, with a corresponding diminution 
of the volume of the jelly. 

Summary 

When gelatine is immersed in a dilute solution of an acid 
combination takes place between the gelatine molecules and the 
hydrogen ions, resulting in the formation of a highly ionisable 
salt of gelatine, the anion of which in tending to diffuse exerts 
on the jelly mass an outward pull, which, being uniform in all 
directions, produces, according to Hooke's Law, an increase in 
the volume of the jelly proportional to the magnitude of the pull. 
In the case of gelatine immersed in a very dilute solution of a 
highly ionisable acid (say, i grm. of gelatine in 100 c.c. of N/iooo 
hydrochloric acid) almost all the acid combines with the gelatine, 
and we have the simplest type of equilibrium, where, practically, 
x=o, y=o, y+z=z=e, and the concentration of the anion of 
the jeUy is the measure of the outward pull and consequently 
of the increase in volume. In more concentrated acid solution 
(say, I grm. of gelatine in 100 c.c. of N/io hydrochloric acid) 
only a part of the acid combines with the jelly, and we have 
y+z>z>e, but here it is neither the total concentration of 
anion of the jelly nor that of the ionised gelatine salt which is 
the measure of the force producing swelling, -but it is the excess 
of concentration of diffusible ions of the jelly over that of the 
external solution. This quantity e is a direct measure of the 
swelling so long as the swelling does not exceed the elastic limit, 
and offers a complete explanation of the peculiar swelling curve 
obtained by immersing gelatine in increasing concentrations of 
hydrochloric acid (see fig. 128). In the most dilute solutions e 
will increase almost directly with the increasing initial concentra- 
tion of acid, but will approach a maximum as the formation 
of the gelatine monochloride nears completion, and must 
then decrease as x becomes larger, according to the equation 



638 PRINCIPLES OF LEATHER MANUFACTURE 



e=—2x + \/4x^+z^, where z has a hmiting maxhnum value. 
The repression of swelhng by the addition of salt is caused by 
the apparent increase in the value of x produced, which results 
in a diminution of the value of e and consequently in a repres- 
sion of the swelling, this action being assisted to some extent by 
the repression of the ionisation of the gelatine salt. 

In the case of weak acids, like acetic, a greater total concentra- 
tion of the acid is required to produce nearly complete com- 
bination of the gelatine with the acid, because the degree of 
combination is determined by the value of y, which, even in the 
more concentrated solutions, will be small because of the repres- 
sion of the ionisation of the acid by the highly ionisable gelatine 
salt. For this reason the swelling of gelatine in acetic acid 
increases with increasing total concentration of acid, and is not 
repressed by the addition of an excess ; in fact, the swelling 
continues up to a strength of acid of N/i, beyond which solution 
of the gelatine takes place. The somewhat stronger formic 
acid actually shows slight repression, whilst very weak acids, 
such as boric, as would be expected, produce little, if any, swelling. 

In pure water, combination must take place, although probably 
only to a very slight extent, between the gelatine molecules and 
the hydrion of the slightly dissociated water, leaving in the jeUy 
a corresponding excess of hydroxyl ions which tend to diffuse 
outward, causing the jeUy to swell. The presence of sulphites 
in the gelatine and carbonic acid in the water tend, of course, to 
produce a greater swelling than the minimum, which would result 
from pure gelatine and water, difficult, if possible, to obtain. 

Some work has been done on the equilibrium of gelatine and 
alkalies, but solution of the gelatine took place at so low a concen- 
tration of the alkali (at about a;=o-04 for sodium hydroxide at 
20°) that the work could not be carried out to the extent desired. 
Work on hide has shown that the swelling is repressed either by 
the addition of excess of alkali or by the addition of ammonium 
chloride. In the former case the swelling is repressed by the 
increase in the value of x, according to the law derived for acids ; 
in the latter case by bringing the solution back almost to a con- 
dition of neutrality, the gelatine compound being again decom- 
posed. It is probable that the laws governing alkaline swelling 
are the same as those governing acid swelling. 

It will be seen that the laws discussed are quite general, and 
that for any particular sample of gelatine at constant temperature 
any variable can be expressed as a direct function of x, and that 
for all acids the value of K' will be the same, whilst the value of 
K is merely dependent on the degree of ionisation of the gelatine 



PAPERS ON GELATINE EQUILIBRIUM 639 

salt formed. With suitable values for K, K', and k, the laws are 
probably applicable to any protein and any acid or alkali. 

If, as the authors believe, the foregoing theory is not merely 
applicable to gelatine, but, with appropriate constants to the 
colloidal swelling of all proteins, it is obviously of far-reaching 
importance, not merely to the special technology in which it 
originated, but to many physiological and medical problems. It 
is only necessary to allude to the work of Loeb on the fertilisation 
of the Echinus egg by saline solutions, of Fischer on oedema, 
and of Pauli and others who attribute muscular energy to colloidal 
swelling and contraction produced by the alternate action of 
sarcolactic acid and the saline constituents of the blood ; whilst 
many of the problems of plant growth and of the semi -permeability 
of vegetable membranes are probably due to analogous causes ; 
and the laws which regulate the swelling of carbohydrate jellies, 
such as agar-agar, starch, and cellulose itself, demand a similar 
investigation. 



APPENDIX C 

COAL-TAR DYESTUFFS SUITABLE FOR DYEING AND STAIN- 
ING VEGETABLE AND CHROME TANNED LEATHER 

By Mr M. C. Lamb, F.C.vS., Director of the Light Leather Depart- 
ment, Leathersellers' Company's Technical College, Tower 
Bridge Road, London, S.E.i 

Abbreviations of the names of dye manufacturers : — 

A. Alliance Colour and Chemical Co., Radnor Street Works 

Stretford, Manchester. 

B. British Dyestuffs Corporation, Blackley, Manchester. 

C. The Clayton Aniline Co., Ltd., and The Society of 

Chemical Industry in Basle, Clayton, Manchester. 
Ca. John Campbell & Co., 75 Hudson Street, New York, 

U.S.A. 
Co. The Colne Vale Dye and Chemical Co., Ltd., Milnsbridge, 

Hudders field. 

D. E. I. du Pont de Nemeurs & Company Inc., Wilmington, 

Delaware, U.S.A. 
G. J. R. Geigy S.A., Basle, Switzerland. 
H. L. B. HoUiday & Co., Ltd., Huddersfield. 
L. London Dye Manufacturing Co., Ltd., City Mills, High 

Street, Stratford, London, E. 
S. J. B. & W. R. Sharp, Ltd., Lumb Works, Edenfield, 

Lanes. 
Sa. Sandoz Chemical Works, Basle, Switzerland. 
Y. Yorkshire Dyeware and Chemical Co., Ltd., Kirkstall 

Road, Leeds. 

Dyeing 

Single Acid Colours suitable for Dyeing Vegetable-tanned 
Leathers 

Browns 

Acid Brown 79186 (B). Manilla Brown (B). 

„ 79487 (B). Nut Brown D. (B). 
„ B. (C). „ „ Y. (B) 

640 



COAL-TAR DYESTUFFS 6. 


Broi&ns 


— (continued) 


Acid Brown G. (C). 


Resorcine Brown A. Cor 


„ L. 348(C). 


(B). 


„ 1580 (C). 


Resorcine Brown (C). 


„ RN. (G). 


„ G. (G). 


„ L. 600 (S). 


.. A. (B). 


Leather Brown G. (A). 


,. (H). 


„ SX. (A). 


Tan Brown RR. (B). 


Leather Tan Brown (A). 


Light Tan (L). 


Orion Brown G. (S). 


Dark Tan (L). 


k 


Yellows 


Acid Yellow 79210 (B). 


Leather Yellow SX. (A). 


Airedale Yellow (Y). 


Metanil Yellow S. (Ca), 


Azo Yellow L (C). 


Naphthol Yellow S. (C). 


„ (Ca). 


„ S. (Ca). 


Azoflavine C. (C) . 


Tartrazine (C). 


7032 (C). 


„ (Ca). 


Citronine Y. Cone. (B). 


„ S. (H). 


„ R. „ (B). 


Pure (H). 


000. (C). 


„ N. (H). 


Fast Leather Yellow C. (C). 


.- (H). 


4G. (C) 


Tartraphenine (Sa). 


R. (C). 


Quinoline Yellow (C). 


Kiton Yellow R. (C). 


Kiton Fast Yellow G. (C). 


„ S. (C). 


„ 2G. (C) 


„ SR. (C). 


„ 3G. (C) 


., G. (C). 


,. R. (C). 


„ 2G. (C). 






Oranges 


Aceko Orange IL (Ca). 


Orange 1 1. (C). 


Acid Orange G. (B). 


„ II. (Sa). 


Crocein Orange SX. (A). 


„ II. (A). 


Kiton Fast Orange G. (C). 


„ II. (S). 


„ 2R. (C). 


„ MNO. (C). 


Orange IL (H). 


„ R. (C). 


., II. (B). 






Scarlets 


iVceko Scarlet R. (Ca). 


Leather Scarlet (A). 


„ 2R. (Ca). 


Ponceau R. (H). 


„ 3R. (Ca). 


„ 2R. Cone. (H). 




41 



642 PRINCIPLES OF LEATHER MANUFACTURE 



Scarlets — (continued) 



Aceko Scarlet 4R. (Ca). 
„ 2G. (Ca). 
Coomassie Milling Scarlet G. 

(B). 
Coomassie Scarlet 9012K. (B). 
Croceine Scarlet 9187K. (B). 

„ (Ca). 
Ponceau 4R. (H). 



Ponceau SS. (L). 
Scarlet R. (B). 

„ 2R. (B). 

,. 3R. (B). 

„ 50 (B). 

„ 2R. (S). 
Milling Scarlet 5B. (B). 



Reds 



Aceko Fast Red (Ca). 

Acid Milling Red G. Cone. 

(G). 
Acid Milling Red R. Cone. 

(G). 
Acid Magenta NS. (Co), 
Acid Red 903 (S). 
Acid Rhodamine R. (C). 
2R. (C). 
3R (C). 
Brilliant Sulphon Red B. (Sa) 
Cardinal Red J. (B). 

„ 3B. (B). 
Claret Red (Ca). 



Fast Leather Red PSNR. (C). 
„ C. (C). 
„ Red A. (A). 
„ „ A. Ex. (A). 
„ „ S. Ex. (A). 
„ „ A. Ex. (S). 
,. „ EAS. (B). 
Kiton Red S. (C). 
>, „ G. (C). 
., 6B. (C). 
Roccelline (C). 

„ (Ca). 

Toney Red B.D. (B). 



Azo Rubine (Sa). 
„ (Ca). 
Benzyl Bordeaux B. (C). 
Bordeaux B. (A). 
Brilliant Bordeaux B. (H). 
Carmoisine (Ca). 



Bordeaux 

Carmoisine Cone. (Sa). 

Erio Fast Fuchsine BL. Cone. 

(G). 
Helian thine G. (G). 
Jasmine (G). 
Pure Bordeaux B., (B). 



Blues 



Aceko Blue CY. (Ca). 
Benzyl Blue S. (C). 
„ „ B. (C). 
Fast Acid Blue RH. (B). 
Fast Leather Blue L. (C). 
„ AL (C). 
„ ARL (C). 
Liduline (C). 



Kiton Fast Blue V. (C). 
Leather Blue RR. (A). 

„ SX. (A). 

„ BB. (A). 
Marine Blue (H). 
Orion Blue (S). 
Soluble Blue NS. (Co). 
.. A. (A). 



COAL-TAR 


DYESTUFFS 








Blues- 


-(continued) 






Induline BL. (Co). 




Soluble Blue 


(Ca) 




Crystals L. 


332 (S). 


,, „ 


3M. 


"(B) 


„ A. (B). 




Solid Blue B 


(G) 




„ 2B. (B). 




Xylene Blue VS. 


(Sa) 


.. 5B (B). 




■• 







643 



Greens 



Acid Green B. Ex. (C). 
„ G. Ex. (C). 
Alizarine Cyanine Green F. 

Powder (B). 
Azo Dark Green A. (G). 
Benzyl Green B. (C). 
Direct Green L. 854 (S). 
Erioglaucine A. (G). 
Ex. (G). 



Erio Green B. (G). 
Kiton Fast Green V. (C). 
Leather Green SX. (A). 
Lissamine Green B. (B). 
Naphthol Green (Ca). 

„ 9211K. (B). 
New Fast Acid Green (Ca), 
Wool Green S. (Sa). 
„ S. (C). 



Violets 



Aceko Violet 4B. (Ca). 

,, loB. (Ca). 
Acid Violet 6B. (G). 

4BNS. (Sa). 
3BN. (C). 
4BN. (C). 
6BN. (C). 
7B. (C). 
Benzyl Violet 5R. (C). 
„ 4B. (C). 
„ 5BN. (C). 



Benzyl Violet 6B. (C). 

„ loB. (C). 
Coomassie Violet R. (B). 
Direct Violet L. 723 (S). 
Fast Leather Violet 4R. (C). 
Fast Sulphon Violet 5BS. 

(Sa). 
Kiton Fast Violet loB. (C). 
Leather Violet (L). 
Victoria Violet (C). 



BUte-Blacks and Blacks 



Acid Black 4BNN. (C). 
„ „ HA. (C). 
„ „ PR. (C). 
„ ., PB. (C). 
„ (Ca). 
Coomassie Black DW. (B). 
Fast Sulphon Black F. (Sa). 
Naphthalene Black 12B. (B). 
„ B. (B). 
„ BD. (B). 
Naphthol Blue Black (C). 

„ B. Cone. (G). 



Naphthol Black ESN. (S). 
Nigrosine G. Crystals (B). 

B. Cone. (S). 

ES. Crystals (S). 

Crystals W. (Co). 

S. (A). 

B. (A). 

A. (C). 

B. (C). 
K. (C). 
S. 1471 (C). 



644 PRINCIPLES OF LEATHER MANUFACTURE 



Single Basic Dyes suitable for [Dyeing Vegetable-tanned 
Leathers 

Browns 



Basic Brown BXN. (D). 




Canelle 3373 (C). 


i> 


„ GX. (D). 




.. 5733 (Q. 


,, 


„ 1557 (C). 




„ 8337(C). 


Bismarck Brown R. loos 


(B). 


„ 63N. (C). 




„ G. (C). 




„ OF. (C). 






. R. (C). 




„ ES. (C). 






. M. (C). 




Chocolate Brown 3936 (C). 






, 8634 (S). 




„ MNB. 86(C). 






. (A). 




Havanna 352 (C). 






, R. Cone. 
. R. (H). 


(H). 


Leather Brown A. (C). 
„ G. (C). 






, G. (H). 




„ 135 (C). 






R. Cone. 


(Co). 


,. 3378(C). 






, Y. (Ca). 
, R. (Ca). 




„ 4183 (C). 
„ 8669 (C). 


Brown A.T. (G). 




Vesuvine GS. (Sa). 


Canelle 1352 (Cy 










Yellows 


Acridine Yellow R. (H). 




Brilliant Phosphine 2G. (C). 


, 


„ 2R. (H). 




5G. (C). 


Auromine 0. (Sa). 




R. (C). 




, 0. (H). 




Patent Phosphine G. (C). 




0. Cone. (H). 




2G. (C). 




„ II. (H). 




„ M. (C). 




0. (B). 




„ R. (C). 




, II. (C). 




GGMK.(C.) 




. 0. (C). 




Saba Phosphine G. (Sa). 




00. pure (C). 




2G. (Sa). 




, 0. (G). 




Sella Flavine G. (G). 




, 0. (D). 




„ R. (G). 




Cone. (D). - 




Sella Brilhant Yellow P. 




, (Ca). 




Cone. (G). 


B 


rilliant Phosphine G 


(C). 


Tannoflavine T. (Sa). 






Oranges 


Acric 


ine Orange L. (B) 




Chrysoidine R. (D). 


Chrys 


ioidine YRP. (B). 




1606 (S). 




GS. (Sa). 




G. (D). 




R. (G). 




(Ca). 




, 


R. (A). 




Pure Chrysoidine YD. (Co). 



COAL-TAR DYESTUFFS 
Reds 



645 



Magenta 87952 (B). 

.> (C). 
Russian Red B. (C). 
„ „ G. (C). 
Rhodamine B. (C). 
G. (C). 
6G. (C). 
B. (D). 



Basic Marine Blue (Ca). 
Bengal Blue R. (G). 
Fast Blue 3R. (Sa). 
Methylene Blue 2B. (B). 

ZF. (B). 

BBH. (H). 

BB. Ex. (H). 

R. Ex. (H). 

2B. (Sa). 

D. (G). 



Rhodamine B. Ex. (D). 
Safranine GOOO. (C). 

„ MN. (C). 

„ BS. (H). 

„ RFF. (H). 

,. Y. (Ca). 
Tannin Pink C. (B). 



Blues 



Methylene Blue XD. (D). 
„ B. (D). 

„ 1565 (S). 

,, „ (Ca). 

Turquoise Blue G. (B). 
Victoria Blue BX. (D). 

„ Cone. (D). 

„ B. Base (D). 

„ B. (Ca). 



Greens 



Basic Green B. Cone. (H). 
„ G. Cone. (H). 
Fast Green O. (C). 

„ „ YYO. (C). 
Malachite Green Crystals A. 

(B). 
Malachite Green Crystals (Ca). 
Methylene Green B. (H). 



Methylene Green G. (H). 
„ P. (C). 
„ „ G. Ex. (Sa). 

.. (G). 
Victoria Green Small Crystals 

(D). 
Victoria Green B. Powder 
(D). 



Violets 



Crystal Violet Powder (D). 

„ Base (D). 
Methyl Violet NE. (D). 

6B. (Ca). 

2B. (Ca). 

4B. (Ca). 

(A). 

2B. Cone. (S). 

BE. (G). 



Methyl Violet 6B. Cone. (H). 

„ 2B. Cone. (H). 

„ 2B. (B). 

„ loLB. (B). . 
Violet RN. (C). 
.. R. (C). 

„ 3R.(C). 

,. B. (C). 
,, 5BO. (C). 



646 PRINCIPLES OF LEATHER MANUFACTURE 



French Black 67445 (H). 
Jute Black G. Cone. (H). 
Leather Black A. (Sa). 
„ CII. (C). 



Blacks . 



Leather Black SM. (C). 

„ CBD. (C). 
Nigeria Black BX. (D). 

,. GX. (D). 



Staining 

Single Acid Colours suitable for Staining Vegetable-tanned 
Leathers 

Browns 



Acid Brown 79186 (B). 


Leather Tan Brown (A). 


„ B. (C). 


Orion Brown G. (S). 


,, „ G. (C). 


Resorcine Brown A. Cone. 


„ L. 348(C). 


(B). 


„ 1580(C). 


Resorcine Brown (C). 


„ RN. (C). 


„ G. (G). 


„ L. 600 (S). 


Tan Brown RR. (B). 


Leather Brown G. (A). 


Light Tan (L). 


„ SX. (A). 


Dark Tan (L). 




Yellows 


Acid Yellow 79210 (B). 


Citronine 000. (C). 


Airedale Yellow (Y). 


Leather Yellow SX. (A). 


Azo Yellow I. (C). 


Metanil Yellow S. (Ca). 


,. (Ca). 


Naphthol Yehow S. (C). 


Azoflavine C. (C). 


„ S. (Ca). 


7032 (C). 


Tartraphenine (Sa). 




Oranges 


Aceko Orange II. (Ca). 


Orange II. (A). 


Acid Orange G. (B). 


„ II. (Sa). 


Crocein Orange SX. (A). 


„ II. (S). 


Orange II. (H). 


„ MNO. (C). 


„ II. (C). 


„ R. (C). 




Scarlets 


Aceko Scarlet R. (Ca). 


Coomassie Scarlet 9012 K. (B) 


„ 2R. (Ca). 


Leather Scarlet (A). 


„ 3R- (Ca). 


Scarlet 50 (B). 


„ 4R. (Ca). 


„ 2R. (S). 


„ 2G. (Ca). 


Croceine Scarlet (Ca). 



COAL-TAR DYESTUFFS 



647 



Reds 



Aceko Fast Red (Ca). 

Acid Milling Red G. Cone. 

(G). 
Acid Milling Red R. Cone. 

(G). 
Acid Magenta NS. (Co). 
Acid Red 903 (S). 
Brilliant Sulphon Red B. (Sa). 
Cardinal Red 3B. (B). 
Claret Red (Ca). 



Fast Leather Red PSNR. (C). 
C. (C). 
Red A. (A). 
„ A. Ex. (A). 
„ S. Ex. (A). 
„ A. Ex. (S). 
„ EAS. (B). 
Roccelline (C). 
„ (Ca). 



Bordeaux 



Azo Rubine (Sa). 
„ ,. (Ca). 
Benzyl Bordeaux B. (C). 
Bordeaux B. (A). 
Brilliant Bordeaux B. (H). 
Carmoisine (Ca). 



Carmoisine Cone. (Sa). 
Erio Fast Fuchsine BL. 

(G). 
Helian thine G. (G). 
Jasmine (G). 
Pure Bordeaux B. (B). 



Cone. 



Blues 



Aceko Blue CY. (Ca). 
Benzyl Blue B. (C). 
„ S. (C). 
Disulphine Blue 87724 (B). 
Fast Leather Blue L. (C). 
„ AL (C). 
ARL (C). 
Induhne (C). 

BL. (Co). 

Crystals L. 332 (S). 
„ A. (B). 



Induline 5B. (B). 
Leather Blue RR. (A). 

„ SX. (A). 

„ BB. (A). 
Marine Blue (H). 
Orion Blue (S). 
Soluble Blue 3B. (B). 

„ NS. (Co). 

„ A. (A). 

„ (Ca). 
Sohd Blue M. (G). 



Greens 



Acid Green B. Ex. (C). 
„ G. Ex. (C). 
„ ,. (B). 
Azo Dark Green A. (G). 
Benzyl Green B. (C). 
Direct Green L. 854 (S). 
Erioglaucine A. (G). 
Ex. (G). 



Erio Green B. (G). 
Leather Green SX. (A). 
Lissamine Green B. (B). 
Naphthol Green (Ca). 

„ 9211K. (B). 
New Fast Acid Green (Ca). 
Wool Green S. (Sa). 
„ „ S. (C). 



648 PRINCIPLES OF LEATHER MANUFACTURE 



Violets 



Aceko Violet 4B. (Ca). 

„ „ loB. (Ca). 

Acid Violet 6B. (G). 

4BNS. (Sa). 

3BN. (C). 

4BN. (C). 

6BN. (C). 

7B. (C). 
Benzyl Violet 5R. (C). 
» 4B. (C). 



Benzyl Violet 5BN. (C). 

„ 6B. (C). 

„ loB. (C). 
Coomassie Violet R. (B). 
Direct Violet L. 723 (S). 
Fast Leather Violet 4R. (C). 
Fast Sulphon Violet 5BS. (Sa) 
Leather Violet (L). 
Victoria Violet (C). 



Blue-Blacks and Blacks 



>> >> 



)) >> 



Acid Black 4BNN. (C). 
., ,. HA. (C). 
PR. (C). 
PB. (C). 
(Ca). 

Fast Sulphon Black F. (Sa). 
Naphthalene Black 12B. (B). 
. „ „ B. (B). 

„ BD. (B). 
Naphthol Blue Black (C). 
Naphthol Blue Black B. 
Cone. (G). 



Naphthol Black ESN. (S). 
Nigrosine G. Crystals (B). 
B. Cone. (S). 
,, ES. Crystals. 

Crystals W. (Co). 
„ S. (A). 
„ B. (A). 
„ A. (C). 
„ B. (C). 
., K. (C). 

S. 1471 (C). 
„ SS. (B). 



Single Basic Dyes suitable for Staining Vegetable-tanned 
Leathers 

Browns 



Basic Brown BXN. (D). 




Canelle 3373 (C). 


>> >> 


GX. (D). 




,. 5733 (C). 


>) >> 


1557 (C). 




., 8337(C). 


Bismarck Brown R. iocs 


(B). 


,. 63N. (C). 


»> > 


. G. (C). 




„ OF. (C). 


>> > 


, R. (C). 




„ ES. (C). 


t> > 


, M. (C).. 




Chocolate Brown 3936 (C). 


}> > 


, 8634 (S). 




„ MNB.86(C) 


)> » 


. (A). 




Havanna 351 (C). 


>> > 


, R. Cone. 


(H). 


Leather Brown A. (C). 


>» > 


, R. (H). 




„ G. (C). 


>> > 


, G. (H). 




.. 135(C)'. 


>> > 


, R. Cone. 


(Co). 


>, 3378 (C). 



COAL-TAR DYESTUFFS 



649 



Browns— (contiimed) 



Bismarck Brown Y. (Ca). 

„ R. (Ca). 
Brown A. T. (G). 
Canelle 1352 (C). 

Acridine Yellow R. (H). 

„ 2R. (H). 
Aiiromine 0. (Sa). 

O. (H). 

O. Cone. (H). 

IL (H). 

O. (B). 

II. (C). 

O. (C). 

00. pure (C). 

O. (G). 

O. (D). 

Cone. (D). 

(Ca). 
Brilliant Phosphine G. (C). 



Aeridine Orange L. (B). 
Chrysoidine YRP. (B). 

GS. (Sa). 

R. (G). 

R. (A). 



Magenta 87952 (B). 

» (C). 
Russian Red B. (C). 
„ G. (C). 
Rhodamine B. (C). 
G. (C). 
6G. (C). 
B. (D). 



Basic Marine Blue (Ca) 
Basic Blue (C). 
Bengal Blue R. (G). 



Leather Brown 4183 (C). 
„ 8669 (C). 
Vesuvine GS. (Sa). 



Yellows 



Brilliant Phosphine 2G. (C). 
„ 5G. (C). 
,. • R. (C). 
Patent Phosphine G (C) 
2G. (C). 
M. (C). 
„ R. (C). 

GGMK.(C). 
Saba Phosphine G. (Sa). 
2G. (Sa). 
Sella Flavine G. (G). 
„ R. (G). 
„ BriUiant Yellow P. 
Cone. (G). 
Tannoflavine T. (Sa), 



Oranges 



Chrysoidine R. (D). 

1606 (S). 

G. (D). 

(Ca). 
Pure Chrysoidine YD. (Co) 



Reds 



Rhodamine B. Ex. (D). 
• Safranine GOOO. (C). 

„ MN. (C). 

„ BS. (H). 

„ RFF. (H). " 

.. Y. (Ca). 
Tannin Pink C. (B). 



Blues 



Methylene Blue XD. (D). 
„ B. (D). 



650 PRINCIPLES OF LEATHER MANUFACTURE 



Blues — (continued) 



Fast Blue 3R. (Sa). 
Methylene Blue 2B. (B). 

ZF. (B). 

BBH. (H). 

BB. Ex. (H). 

R. Ex. (H). 

2B. (Sa). 

D. (G). 



Methylene Blue 1565 (S). 
„ (Ca). 
. G. (C). 
Turquoise Blue G. (B). 
Victoria Blue BX. (D). 
„ Cone. (D). 
„ B. Base (D). 
„ B. (Ca). 



Greens 



Basic Green B. Cone. (H), 
„ G. Cone. (H). 
Fast Green 0. (C). 

„ YYO. (C). 
Malachite Green Crystals A. 

(B). 
Malachite Green Crystals (Ca) 
Methylene Green B. (H). 



Crystals Violet Powder (D). 

„ Base (D). 
Methyl Violet NE. (D). 

6B. (Ca). 

2B. (Ca). 

4B. (Ca). 

(A). 

2B. Cone. (S). 

BE. (G). 



French Black 67445 (B), 
Jute Black G. Cone. (H). 
Leather Black A. (Sa). 

„ CII. (C). 

„ 1722 (C). 



Methylene Green G. (H). 

„ P. (C). 

„ G. Ex. (Sa). 

,. (G). 
Victoria Green Small Crystals 

(D). 
Victoria Green B. Powder (D). 



Violets 



Methyl Violet 6B. Cone. (H). 

„ 2B. Cone. (H). 

„ 2B. (B). 

„ loLB. (B). 
Violet RN. (C). 
., R. (C). 

„ 3R. (C). 

„ B. (C). 
„ 5BO. (C). 



Blacks 



Leather Black SM. (C). 

„ CBD. (C). 
Nigeria Black BX. (D). 

„ GX. (D). 



Acid Mixtures suitable for Dyeing and Staining Broivns 
upon Vegetable-tanned Leathers 



Resorcine Brown A. Cone. (B). 
Acid Yellow 79210 (B). 
Naphthol Green 9211K. (B). 



Dark Tan (L). 
Azofiavine C. (C). 
Acid Green G. Ex. (C). 



COAL-TAR DYESTUFFS 651 

Acid Mixtures suitable for Dyeing and Staining Browns 
upon Vegetable-tanned Leathers — continued 

Manilla Brown (B). Orange II. (H). 

Citronine R. Cone. (B). Tartrazine (H). 

Wool Green S. (Sa). Kiton Fast Green V. (C). 

Resorcine Brown G. (G). 
Leather Yellow SX. (A). 
Erioglaucine A. (G). 

Basic Mixtures suitable for Dyeing and Staining Browns 
upon Vegetable-tanned Leathers 

Bismarck Brown 8634 (S). Chrysoidine R. (G). 

Chrysoidine 1606 (S). Auromine O. (G). 

Fast Blue 3R. (Sa). Bengal Blue R. (G). 

Bismarck Brown G. (C). . Bismarck Brown 8634 (S). 

Canelle 1352 (C). Chrysoidine 1606 (S). 

Fast Green O. (C). Direct Green L. 854 (S). 

Bismarck Brown R. (B). Bismarck Brown R. Cone. 

Auromine O. (B). (Co). 

Malachite Green Crystals A. (B). Pure Chrysoidine YD. (Co). 

Malachite Green Crystals A. 

Vesuvine GS. (Sa). (B). 
Auromine 0. (Sa). 
Fast Blue 3R. (Sa). 

Chrome Leather 

The following dyestuffs are suitable for dyeing chrome leather. 
The leather, after tanning, is washed, neutralised with borax, 
sodium bicarbonate, or washing soda, re-washed, and then mor- 
danted by drumming or paddling in a tannin solution. 

The following are recommended proportions for various shades 
of colour : — 

Dark brown shades, 3 per cent, gambler, i per cent, fustic 

extract. 
Medium brown shades, 2 per cent, gambler, 2 per cent, fustic 

extract. 
Light brown shades, 2 per cent, gambler, 3 per cent, fustic 

extract. 

The leather is dyed immediately after mordanting ; the fat -liquor- 
ing operation is best done after the dyeing has been completed. 



652 PRINCIPLES OF LEATHER MANUFACTURE 

Dyes suitable for dyeing Chrome-tanned Leathers 

Acid Colours, dyed in a weak sulphuric or formic acid solution 

Browns 



Acid Brown RN. (G). 

79186 (B). 

79487 (B). 

B. (C). 

G. (C). 

L. 348 (C). 

1580 (C). 
Manilla Brown (B). 



Nut Brown D. (B). 
„ „ Y. (B). 

Resorcine Brown N. (G). 

A. Cone. 
(B)._ 
Resorcine Brown (C). 
„ G. (G). 
„ A. (B). 



Acid Yellow 79210 (B). 
Acid Orange G. (B). 
Airedale Yellow (Y), 
Azoflavine C. (C). 



Yellows 



Azoflavine 7032 (C). 
Leather Yellow SX. (A). 
Tartrazine (C). 

„ S. (H). 



Reds and Maroons 



Benzyl Bordeaux B. (C). 
Cardinal Red J. (B). 

„ 3B. (B). 
Fast Leather Red C. (C). 
Kiton Red S. (C). 



Kiton Red (C). 

„ „ 6B. (C). 
Pure Bordeaux B. (C). 
Roccelline (C). 



Blacks, mordant with 3 per cent, hematine 



Acid Black 4BNN. (G). 
„ „ HA. (C). 
„ PR. (C). 
„ „ PB. (C). 
„ ., (Ca). 
Coomassie Black DW. (B). 
Fast Sulphon Black F. (Sa). 
Naphthalene Black 12B. (B). 
„ B. (B). 
„ BD. (B). 
Naphthol Black ESN. (S). 



Nigrosine G. Crystals (B). 
B. Cone. (S). 
ES. Crystals (S). 
Crystals W. (Co). 
S. (A). 
B. (A). 

A. (C). 

B. (C). 
K. (C). 
S. 1471 (C). 



COAL-TAR DYESTUFFS 



653 







Basic 


Dyes 






Browns 


Basic Brown 


BXN. (D). 




Canelle 1352 (C). 


,, ,, 


GX. (D). 




„ 3373 (C). 


,, ,, 


1557 (Q. 




,, 5733 (C). 


Bismarck Brown R. loos 


(B). 


„ 8337(C). 


)> > 


, G. (C). 




„ 63N.(C). 


>! ) 


, R. (C). 




„ OF. (C). 


J) > 


, M. (C). 




„ ES. (C). 


>• J 


, 8634 (S). 




Leather Brown A. (C). 


,, , 


, (A). 




„ G. (C). 


>t J 


, R. Cone. 


(H). 


.. 135 (C). 


1> > 


, R. (H). 




,. 3378 (C) 


>> ) 


, G. (H). 




„ 4183 (C) 


>> 1 


, R. Cone. 


(Co). 


„ 8669 (C) 


1) > 


, Y. (Ca). 




Vesuvine GS. (Sa). 


>> > 


. R. (Ca). 




., R. (Sa). 



Yellows 
BriUiant Phosphine G. (C). ^ Patent Pliosphine M. (C). 



2G. (C). 
.. 5G. (C). 
„ R. (C). 
Patent Phosphine G. (C). 
2G. (C). 



„ R. (C). 

GGMK.(C). 
Saba Phosphine G. (Sa). 
2G. (Sa). 
Tannoflavine T. (Sa). 



Blacks, mordant with 3 per cent, hematine 
French Black 67445 (B). Leather Black SM. (C). 



Leather Black A. (Sa). 
„ CIL (C). 



CBD. (C). 



Direct Dyes, dyed in a weak acetic acid bath 
Browns 



Chrome Leather Brown G. 98 

(C). 
Chrome Leather Brown R. 99 
•(C). 

Chlorazol Brown G. (B). 
Chlorazol Brown GN. Ex. 

(B). 
Chlorazol Brown RN. Ex. 

(B). 



Chlorazol Brown HX. (B). 
Dianol Dark Brown BM. (B). 

„ Brown GM. (B). 
Diphenyl Brown GR. supra 

(G). 
Omega Chrome Brown G. 

(Sa). 
Omega Chrome Brown P. 

(Sa). 



654 PRINCIPLES OF LEATHER MANUFACTURE 

Yellows and Oranges 



Afghan Yellow GX. (B). 
Alizarine Yellow G. (Sa). 
,, Orange R. (Sa). 
Chlorazol Fast Yellow B. (B). 
„ - R. (B). 
NX. (B). 
FG. (B). 
,, Orange R. Ex. (B). 



Chrysophenine G. (B). 
Diphenyl Chlorine Yellow 

FF. (G). 
Dianol Fast Orange G. (B). 
Polyphenyl Yellow R. (G). 
Polyphenyl Orange R. Ex. 

(G). 
Sun Yellow 3G. (G). 



Reds and Maroons 



Benzo Purpurine 4B. (B). 
Chicago Red (G). 
Chlorazol Pink Y. (B). 
Dianol Fast Red F. (B). 



Dianol Fast Pink BK. (B). 

„ Fast Red K. (B). 
Diphenyl Fast Red B. (G). 
Jasmine (G). 



Greens and Blues 



Brilliant Delphine Blue B. 

(Sa). 
Chlorazol Sky Blue GW. (B). 
FF. (B). 
Blue 3B. (B). 
Green B. (B). 



Chlorazol Dark Green PL. (B) , 
Diphenyl Deep Blue R. (G). 
„ Green KGW., supra 
(G). 
Diphenyl Blue KF. (G). 
Fast Blue 3R. (Sa). 



Dark Green PL. (B). Gallocyanine (Sa). 



Violets 



Chlorazol Violet WBX. (B). 



Dianol Violet R. (B). 



Blacks 



Dianol Black BH. (B). 
Chlorazol Black E. Ex. (B). 

„ DV. (B). 
Chrome Leather Deep Black 

G. Cone. (C). 
Chrome Leather Deep Black 
C. Cone. (C). 



Chrome Leather Black E. (C). 
E. Ex. 

(C). 
Chrome Leather Black E, 

Ex. Cone. (C). 



COAL-TAR DYESTUFFS 655 

Mordant Dyestuffs 
Yellows and Browns 

Alizarine Orange M. Paste Chrome Fast Brown A. (C). 
(B). „ „ G. (C). 

Alizarine Brown M. Paste ,, ,, PC. (C). 

(B). „ „ R. (C). 

Alizarine Yellow G. (Sa). ,, ,, 537 (C). 

,, R. (C). Chrome Leather Brown 2G. 
Chrome Fast Yellow 2G. (C). (C). 

5G. (C). Khaki Yellow WN. Paste (B). 



APPENDIX D 

THE DETERMINATION AND CONTROL OF ACIDITY 
IN TAN LIQUORS 

W. R. Atkin and F. C. Thompson 

The importance of the acidity of tan liquors is shown by the 
ample literature on the subject, describing for the most part 
work which has not achieved its aim, namely, the actual measure- 
ment of the swelling power of tan liquors. At the outset we may 
state our opinion that acidity, even when the presence of salts 
of weak acids is taken into account, is not the only factor govern- 
ing swelling. Two factors, the influence of which is obscure, 
are : {a) the astringency and actual tanning effect of the tannins 
present, and {b) the effects of neutral salts of strong acids such 
as sodium chloride, sodium sulphate, etc. A well- conceived and 
direct method of arriving at the effect of astringency, etc., is that 
of Claflin, who carries out swelling experiments with hide-powder 
and the liquor under investigation, where the amount of liquid 
absorbed by the hide-powder measures the swelling power. The 
influence of neutral salts has been well brought out in numerous 
papers by J. A. Wilson and his collaborators, A. W. Thomas and 
M. E. Baldwin, and by J. W. McBain. The effect of adding sodium 
chloride to dilute hydrochloric acid is to raise the hydrion con- 
centration as measured by the hydrogen electrode, whereas 
sodium sulphate added to weak acids has an opposite effect. 

McBain found that the partial vapour pressure of acetic acid 
in dilute solution was greatly augmented in the presence of 
sodium chloride. These interesting effects are believed by Wilson 
to be intimately connected with the degree of hydration of the 
added ions, whereas McBain speaks of " enhanced chemical 
potential." Whatever the cause the effect is of great importance, 
particularly in chrome tanning. 

In this paper, however, we are concerned only with a direct 
determination of acidity. All previous attempts have depended 
upon titration of the tan liquors with alkali. H. R. Procter 
and R. A. Seymour- Jones published in 1910 a comprehensive 
review of the earlier work on this subject, and in addition described 

656 



ACIDITY IN TAN LIQUORS 657 

several variations involving the use of indicators. The two main 
difficulties, however, were : 

(a) The colour of the tan liquors masked the colour of the 

indicator ; 

(b) Except in a neutral atmosphere there was always a con- 

siderable oxidation and darkening on the addition of 
alkali. 

Attempts have been made to overcome the first difficulty by 
detannisation previous to titration, but this procedure causes 
serious errors, as in almost all cases there is a marked co-precipita- 
tion of acid. This objection applies to the present official method 
of the A.L.C.A., in which detannisation is effected by means of 
an alcoholic solution of gelatine. Stiasny, however, detannises 
with a mixture of hydrochloric acid and formaldehyde, and in 
this case there appears to be no loss of acid, but the method can 
only be applied to catechol tans. 

The question now to be discussed is what acidity should be 
measured. It is necessary to define at the outset what is meant 
by " acidity," and to show that measurement of total content 
in acid, even if this could be accurately performed, would not give 
the information which is desired by the sole leather tanner. A 
concrete example may make this point clear. Consider the two 
following cases : {a) 25 c.c. of N/10 acetic acid, and (b) 25 c.c. 
of N/io acetic acid containing 0-205 grm. of sodium acetate, 
and consequently also Njio in this substance. If these two 
solutions were titrated with alkali, using phenolphthalein as 
indicator, the same result would be obtained in each case, but if 
an attempt were made to titrate, using methyl orange, it would 
be found that mixture (&) was already alkahne. In harmony 
with this, the swelling power on water-swollen gelatin of solution 
[b) would be nil, whereas {a) would produce a pronounced increase 
of swelling. The explanation is found in a consideration of the 
ionisation phenomena in the two cases. 

As is well known, the property of acidity is due to the presence 
in solution of hydrogen ions. An acid in solution dissociates 
into ions according to the following scheme : 

HAc->H- + Ac'. 

In the case of weak acids the extent of this dissociation, i.e. 
the concentration of ions produced, is in accordance with the well- 
known law of mass-action. 

This states that under given circumstances of original concen- 
tration and temperature the concentrations of the products of 
reactions at equilibrium are, when multiplied together, in strict 

42 



658 PRINCIPLES OF LEATHER MANUFACTURE 

proportion to the concentrations of reacting substances, also 
multiplied together. In other words, if A and B react together 
to form X and Y, then at equihbrium [A] x [B] = K[X] X [Y]'. 

The square brackets indicate concentrations, and K is a number 
termed the equilibrium constant of the reaction. 

Consider from this point of view the electrolytic dissociation 
or ionisation of a weak acid, i.e. one which is only ionised to a 
slight extent. Here we only have one reacting substance, namely, 
the acid which is partially split up into the two reaction products, 
the kation (H-) and the anion (Ac') — 

K[HAc]=[H-]x[Ac']. 

Here K is the dissociation or ionisation constant, and in the 
case of weak acids is always very small, e.g. o-ooooi8 for acetic 
acid. The square brackets indicate concentration in terms of 
normality, i.e. for N/io acetic acid, of which over. 98 per cent, 
remains undissociated, [HAc]=o-i, approximately, being actually 
a little less. 

Whatever other ions may be present in solution at the same 
time as those of acetic acid, the above equation holds good. 
Consequently, if the concentration of the anion [Ac'] be increased 
by some means without increasing the concentration of undis- 
sociated acid [HAc], then in order to maintain the equality the 
concentration of the kation [H-] must diminish. In other words, 
the acidity is diminished. In the case of decinormal acetic acid 
we have 

K[CH3COOH]=[H-]x[CH3COO']=[H-]2, 

since hydrogen and acetate ions are necessarily equal in number. 
Since the acid is decinormal and almost all undissociated, we have 
approximately 

o-iK=[H-]2 

or [H-] = Vo-oooooi8 

=o-ooi34=i-34X io~^ 

The effect of adding sodium acetate is to increase largely the 
number of acetate ions. Usually the salt of even a weak acid 
is largely dissociated, and in extending the calculation to the case 
of mixture (b) we shall assume as an approximation that sodium 
acetate is completely ionised in decinormal solution. Thus 
[Ac'] now=o-i, whilst [HAc] is unaffected. The sodium ions 
produced do not affect the case. We still have 

K[CH3C00H] = [H-] X [CH3COO'], 
but now o-i K=[H-] xo-i 

or [H-]=K=-ooooi8=i-8xio~5. 



ACIDITY IN TAN LIQUORS 659 

Thus [H-], or the acidity, has been reduced to less than 2 per 
cent, of its value in N/10 acetic acid. The effect of any other 
amount of sodium acetate can be calculated easily. Suppose 
the N/10 acid to be made N/i in sodium acetate. Then ap- 
proximately 

o-i K=[H-]xi-o 

or rH]=— or i-8xio~«. 

■" 10 

More accurate results can be obtained by finding the actual 
percentage dissociation of sodium acetate from the tables and 
using it in the calculation. 

An interesting and important property of mixtures of weak 
acids and their salts is that the [H-] is very little altered by even 
considerable dilution. Such solutions are called " buffers," and 
their behaviour is readily explained in the light of the above 
equations. Consider again the case of mixture {b). We have 
here 

o-iK=[H-]xo-i. 

This formula may now be made more accurate by introducing 
corrections for the not quite complete ionisation of the sodium 
acetate, which in N/10 solution is ionised to the extent of 79 per 
cent, and in N/100 solution to 87 per cent. 

Consequently, for the N/10 solution the above equation becomes 

o-ixK=[H-]xo-ix— , 
100 

or o-ix 1-8 xio~^=[H-]x 0-079 

or [H-]=2-28xio-^ 

Now consider the case where mixture (6) is diluted ten times. 

We now have 

87 
o-oixK=[H-]xo-oix — , 
■* 100 

or o-oix 1-8 xio~5=[H-]x 0-0087 

or [H-]=:2-o7Xio~^. 

In the case of pure acetic acid containing no added sodium 
acetate the [H-] is 1-3x10"^ in N/10 solution and 0-42x10"^ in 
N/ioo solution, representing a drop of nearly 70 per cent., as com- 
pared with about 10 per cent, in the case of mixture {b). 

Now consider the case of pure distilled water which is very 
slightly ionised into hydrions and hydroxyl ions so that 

K[H20]=[H-]x[0H'] and [H-]=[OH']. 

As the mass of undissociated water is very large compared with 



66o PRINCIPLES OF LEATHER MANUFACTURE 

the mass of the free ions we might regard it as also constant, so 
[H-]x[OH'] = Kze;. 

Here Kw is the dissociation constant for pure water, and has 
the very low value at 21° C. of Io~^^ and thus the value for [H-] 
or [OH'] is 10"'^ grm. ions per litre. 

If an acid be added to distilled water the acid is dissociated 
either partially or wholly into hydrions and the anions of the 
acid employed. Thus [H-] in such a mixture is greater than 
10"'^, and the solution is " acid." Suppose acid had been added 
so that [H-] was lo"^, then [OH'] would be lo-^i, for 

[H-]x [OH'] = 10-14. 

From the above it will be understood that a solution is " neutral," 
" acid," or " alkaline " according to whether [H-] has a value 
equal to, greater than, or less than lO"'^ respectively. Obviously, 
if [H-] does not equal 10-'^, then it cannot be equal to [OH']. 

Sorensen, in order to facihtate the plotting of curves, has 
introduced the symbol Pjj to denote what he calls the " hydrogen 
ion exponent," where Pjj is the logarithm to the base 10 of [H-] 
but with the negative sign omitted, or Pjj=— log^o [H-]. For 
example, AT/io acetic acid has [H-] = i-36xiO""^=io°'i3^X io~^ 
= 10-2-867^ so Ph=2-867. 

It should be noticed that Pjj decreases with increasing acidity, 
and that if [H-] be increased tenfold the value of P^ is decreased 
by i-o. 

From the case of acetic acid and sodium acetate mentioned 
above it wiU be seen that Pjj cannot be determined by ordinary 
titration. 

The most accurate method of determining the [H-] or Pj, is by 
means of the hydrogen electrode, and this method has been applied 
by Wood, Sand, and Law in the case of tan liquors. The method 
is an electrical one, requiring a considerable amount of apparatus, 
and does not seem to have become universally popular with 
tannery chemists. 

Other methods depend upon the measurement of the velocity 
of chemical reactions, such as the hydrolysis of cane sugar or 
methyl acetate, where acids are used as catalysts, but these involve 
too much time. The simplest method is by the use of indicators, 
and may be called the colorimetric method. It has the great 
advantage that it is rapid, and does not require any elaborate 
apparatus. 

As is well known, indicators are substances that vary in colour 
with varying acidity or alkalinity of the solution to which they 
are added. This change of colour takes place over a definite 



ACIDITY IN TAN LIQUORS 66i 

range of P^, and at various points within this range the indicator 
shows intermediate tints. The solution is " neutral " to the 
indicator at the mid point of the range. This neutrality does not 
mean that [H-] = [OH'] as in pure water. For example, 
"neutrality" to phenolphthalein occurs about Ph=9, whilst in 
the case of methyl orange "neutrality" is in the region Ph=4, 
the [H-] in the latter case being 100,000 times greater than in the 
case of phenolphthalein. Litmus, however, has its " neutral " 
point at about 6-6 and brom-thymol blue at about 6-8. Table I. 
shows a list of convenient indicators, many of which are of the 
new sulphone phthalein type described by Mansfield Clark, Lubs, 
and Acree. One of them, thymol blue, it will be noticed, has a 
double change, one in fairly strong acid solution, and the other 
like that of phenolphthalein. 

The colorimetric method has been described by various workers, 
including Veley and Sorensen, but the results obtained were 
only rough in the case of coloured fluids. Sorensen also pointed 
out that the presence of proteins vitiated the results with certain 
indicators. This was especially the case with colloid indicators 
such as Congo red. Walpole surmounted the difficulties due to 
the colour of the fluids by matching the solution under investiga- 
tion (which contained the indicator) against a colourless solution 
of standard P^ containing the same amount of indicator observed 
through a layer of the coloured fluid. His method was modified 
by Hurwitz, Meyer, and Osterberg, who used test-tubes held in a 
box, and finally by Cole and Onslow, who have further improved 
this latter device, which is termed a " comparator," by the 
addition of a ground-glass plate, as shown in the diagram (p. 663) . 
For determining the acidity of tan liquors a comparator which 
takes two pairs of tubes is as large as is necessary. The method 
of procedure is as follows : — 

Test'tube A^ contains 10 c.c. of filtered tan liquor, Ag contains 
10 c.c. of a solution of standard Pg together with five or ten drops 
of a suitable indicator, i.e. an indicator which is changing its 
shade distinctly in that region of acidity. Test-tube Bj contains 
distilled water, and B2 10 c.c. of filtered tan liquor and the same 
number of drops of indicator solution as in Ag. If on looking 
through the two pairs of tubes the colours match, then the P^ 
of the tan liquor is that of the standard used in A^. If not, 
other standards of deeper or lighter shade as may be required are 
substituted for that in A^. The matching is usually performed 
in a minute or so, provided that the necessary ranges of standards 
are to hand. The test-tubes must be of colourless glass, and of the 
same diameter. A selection can easily be made by pipetting 



662 PRINCIPLES OF LEATHER MANUFACTURE 









PI ^ 



^ ft ft'^Q 














•" rt 










cent, 
r ce 
J. 


o 


" 


- 


- 


« 








, 0) o 


OJ 








^ ft^ 


ft 








^o 8 


9 


o 


CM 

O 


O 


^^^-^ 


6 


6 


6 


6 


6 6 



03 



ft 



0'H§^^ ^^ O O 0003 



^OlOOMD "^O opo -f 00 O o 

'^r£ ^'^'^ -^yb ^ J^obobONo 

OHM CO-^ U-JMDVO tvcb CO 






gft .^ ^^^'C^^ 
S -^-a -t - § i §11 



^^t^ftQ^Q^ ^ft^ftJtJ 



o 






ft ;:3 • • • 

ft -< ^ 

^^ ^ ^ as loll 



. . a> 






r3 






1 — 1 






^ 


<U 




olet 
lue 
;nol 




t3 


•>^^ 


o 


a; 


-n'o ^ 













ACIDITY IN TAN LIQUORS 



663 



10 c.c. of water into a number of, say, |-inch tubes, which are very 
convenient, and using those which give the same level of water. 
The tubes are best divided thus into sets, marked, and kept 
separately in boxes. The most convenient means of obtaining 
drops of indicator solution of equal size is to use the slow-dropping 
Dreyer pipettes, which have a capillary end about i inch long. 
With these pipettes there is no danger of adding too many drops 



LIGHT 




Screen 



EYE 



accidentally. As a source of light a Nernst lamp proved to be 
excellent, and good results were obtained with an inverted in- 
candescent gas mantle and with an electric lamp of the half- 
watt type. 

Tan liquors have acidities which lie as a rule between Ph=3'0 
and Pg=4-5. Exceptions are often found in the case of sjmthetic 
tannins, which may be so acid as to reach Ph=i-5. It must first 
be emphasised that neutrality from the tanner's point of view 
is the point of minimum swelling, and that this is not when 
Pjj=7, i.e. when [H-] = [OH']. Experiments by Procter, 
Loeb, Michaelis, and by the authors all tend to fix the iso- 
electric point of gelatin, or the point of minimum swelling, at 

Ph=4-5-47- 

The determination of the point of minimum swelling gives us one 
limit for the range of Pj, in the acid standards. It is plainly not 



664 PRINCIPLES OF LEATHER MANUFACTURE 

necessary to go nearer to neutrality than Pjj=4-7. The other 
hmit may be taken as Pjj=2-8 for most purposes. Only synthetic 
or treated tannins are likely to lie outside this range. The 
standards used were mixtures of (i) acetic acid and sodium 
acetate, (2) acid potassium phthalate and hydrochloric acid. 
The best indicator appears to be tetra-brom-phenol-sulphone- 
phthalein or brom-phenol blue. The colour of this indicator 
changes continuously from Pjj=2-8 to Pjj=4-6, though not verj^ 
much near the limits. Brom-phenol blue, however, is dichroic, 
and is best observed through a yellow colour screen which cuts 
off blue rays. A convenient mode of making such a screen is to 
paint transparent parchment with a mixture of o-6 per cent, 
solution of phenol red and ikf /5 solution of potassium dihydrogen 
phosphate (27-23 grams /liter). The preparation of the standards 
is a very simple matter, following the tables given. The solutions 
required are A^/5 acetic acid, N/^ sodium acetate prepared by 
neutralising 200 c.c. Nji caustic soda with N/i acetic acid and 
diluting to i liter, M/5 acid potassium phthalate (40-828 
grams/liter), N/^ caustic soda, and A^/5 hydrochloric acid : — 



Standard P^^ Solutions. 
Series L 

50 c.c. M/5 acid potassium phthalate ; x c.c. A^/5HC1, mixed 
and diluted to 200 c.c. 



Ph. 


X. 


Ph. 


X. 


Ph. 


X. 


2-8 
2-9 

3-0 
3-1 


26-42 

22-8o 
20-32 
17-70 


3-2 
3-3 
3-4 
3-5 


14-70 

11-80 

9-90 

7-50 


3-6 
3-7 

3-8 

3-9 


5-97 
4-30 
2-63 
I -00 






Series II. 







50 c.c. M/^ acid potassium phthalate ; x c.c. A^/sNaOH, mixed 
and diluted to 200 c.c. 

Ph. X. Ph- X. 



4-0 


0-40 


4-4 


7-50 


4-1 


2-20 


4-5 


9-60 


4-2 


370 


4-6 


12-15 


4-3 


5-17 







ACIDITY IN TAN LIQUORS 



665 



Series III. 
X c.c. N/^ sodium acetate + 100— jv c.c. Nj^ acetic acid. 



Ph. 


X. 


100 — a;. 


Ph. 


X. 


100— a; 


3-8 


12 


88 


4-3 


31 


69 


3-9 


15 


85 


. 4-4 


37 


63 


4-0 


18 


82 


4-5 


42-5 


57-5 


4-1 


22 


78 


4-6 


48 


52 


4-2 


26-5 


73-5 


47 


54 


46 



For instance, a solution of Pjj=3-8 may be prepared in either of 
the two following ways : {a) 50 c.c. M/5 phthalate + 2-63 c.c. 
A^/5 hydrochloric acid diluted to 200 c.c, or {b) 12 c.c. N/^ 
sodium acetate +88 c.c. N/$ acetic acid ; 10 c.c. of either of these 
solutions, together with five drops of brom-phenol blue solution, 
is pipetted into a test-tube, corked well, and labelled Pjj = 3-8. 
It will be found convenient when working in the range from 
Pjj=2-8 to Pjj=3-7 to use ten drops of indicator solution in order 
to intensify the colour differences. The whole range required 
should be made up and kept in a test-tube stand, preferably in 
the dark. The authors have found that the colours do not change 
appreciably in the course of a month. These sets of standards 
have been worked out by Clark and Lubs, and have been 
standardised against the hydrogen electrode. 

A number of tannin solutions have been examined, both in the 
above way and by Procter's lime-water method, with the following 
results, which have been arranged in order of increasing acidity : — 

Procter lime- 
water method 
c.c. satd. lime- 
water per 10 c.c. 
tan liquor. 

1-8 
5-6 
6-2 
6-8 
16-2 

17-3 
27-9 

34-5 



Colorimetric Method. 



Tan Liquor. 



No. I 



Ph- 


as normality 


4-3 
4-2 
4-1 


5 X 10-5 

6-3x10-5 

8 X 10-5 


4-1 
3-8 


8x10-5 
1-6x10-4 


3-7 
3-4 
3-1 


2 X 10-4 

4x10-4 
8 X 10-4 



It will be noticed that the lime-water figures are in the same order 
as the acidities, though no exact correspondence is revealed. 
There is no necessary connection between Pjj and the lime-water 
figure. 



666 PRINCIPLES OF LEATHER MANUFACTURE 

If it be required to raise the acidity of a tan liquor, this can 
easily be done by titrating the liquor with a standard solution 
of the acid to be used until its acidity reaches the required Pjj 
as shown in the comparator ; a calculation will then show how 
much commercial acid should be added to the liquor in the pit. 
What, however, is a desirable P^ must be fixed by the tanner 
according to the class of leather he is dealing with. . The best 
way is to note carefully the acidities of liquors which he finds 
satisfactory, and to reproduce these as required. It will be 
obvious that acid is titrated by acid ; no alkali is used, and the 
difficulties due to oxidation and darkening entirely disappear. 

An interesting application of this acidity method is in the case 
of freshly made analytical solutions of tannin. A surprising 
variation in acidity was found, the range being almost as great 
as in used liquors. 

Tanning Material. Range of P„. 

Solid mimosa extract (4 samples) . . 4- 1-4-3 

Sumach (4 samples) ..... 4- 1-4-2 

Liquid mjn-obalan extract (3 samples) . 3'3-3"4 

Sohd quebracho extract (2 samples) . . 4-1 

Liquid quebracho extract (3 samples) . 4-0-4-2 

Sohd chestnut extract (i sample) . . 3-5 

Liquid chestnut extract (2 samples) . , 3'7-3*75 

Wood, Sand, and Law have previously determined the acidity 
of tan liquors directly by the hydrogen electrode, which is the 
standard method, and much more accurate than the method 
described in this paper. However, it has not become popular 
with tannery chemists, and our purpose is to give a method which 
whilst sufficiently accurate is yet simple in execution and easy 
to understand. The electrometric method is no doubt easily 
and quickly carried out when the complicated apparatus is once 
set up, but to understand and appreciate the method of working 
requires a considerable acquaintance with difficult thermo- 
dynamic theory. Once the standard buffer mixtures described 
above (without indicator) have been prepared, it will be found 
convenient to keep them in well stoppered and carefully labelled 
bottles. Thus new colour standards may be quickly prepared by 
adding the indicator to 10 c.c. when needed. 



APPENDIX E 

THE CAUSTIC ALKALINITY OF LIME LIQUORS 

William R. Atkin and John Atkin, M.C. 

In a recent publication by one of us a method was described for 
ascertaining the caustic alkahnity of Hme hquors, but the method 
so described was indirect, and involved four separate determina- 
tions. We have present in lime liquors the following substances 
which constitute alkalinity : — 

[a) Calcium hydroxide. 
(&) Sodium hydroxide. 

(c) Sulphydrates of sodium and calcium. 

[d) Ammonia and, perhaps, amines, but these latter are 

present, if at all, in very small amounts, and may be 
ignored for practical purposes. 
{e) Sodium and calcium salts of the various protein decom- 
position products, such as proteoses, peptones, amino 
acids, and fatty acids, produced from amino acids by 
deaminisation. 

In order to maintain uniformity with our earlier paper on this 
subject the letters {a), {b), (c), {d), and {e) are employed to refer 
to calcium hydroxide, sodium hydroxide, etc., as above, so that 
a + b is the caustic alkalinity. In addition all estimations are 
performed on 25 c.c. portions of the filtered lime liquor, and 
results stated in c.c. iV/io acid or alkah. 

Several methods have been suggested for ascertaining the 
caustic alkalinity directly by one titration, but they proved of 
little value owing to the difficulty of choosing a suitable indicator. 

We have in lime liquors two strong bases, sodium and calcium 
hydroxides, together with a weak base, ammonium hydroxide, 
and the salt of a strong base combined with a weak acid (sodium 
sulphydrate, which is formed from the sodium sulphide used for 
sharpening the lime liquors). It is well known that indicators 
such as phenolphthalein, which have a colour change on the 
alkaline side of true neutrality, cannot be used for the accurate 
titration of ammonia solutions. On the other hand, an indicator 
such as methyl orange, which possesses a colour change well on 
the acid side of true neutrality, would cause to be included in the 

667 



668 PRINCIPLES OF LEATHER MANUFACTURE 

titration not only the caustic alkalinity but all the ammonia, 
hydrosulphide, and also the sodium and calcium salts of the 
decomposition products of the dissolved proteins, and, as Bennett 
points out, this titration with methyl orange estimates the total 
alkalinity. However, by the aid of the ionic theory, and the 
employment of the comparator recently described, it has been 
found possible to determine quickly, and with reasonable accuracy, 
the caustic alkalinity of lime liquors. Consider the case of 
ammonium hydroxide, which is a weak base, and therefore is only 
slightly dissociated into ions in aqueous solution, 

NH^OH-^NH^'+OH'. 

By the law of mass action we have 

[NH,-] [0H- ] 
[NH4OH] • ■ ' ^ ' 

where K is a constant known as the dissociation constant, and 
at 25° C. has a value of 2-3 x lO"^. [NH4-] denotes the concentra- 
tion of ammonium ions, [OH'] the concentration of hydroxyl 
ions, and [NH4OH] is the concentration of the undissociated 
ammonium hydroxide. As [NHi-] or [OH'] is relatively very 
small compared with [NH4OH], we may regard the latter as 
constant, so that the above equation may be simplified to 

[NH4-]x[OH']=constant . . . (2) 

If, now, a considerable quantity of NHj- ion be added to a solution 
of ammonia as an ammonium salt, e.g. ammonium chloride, which 
is almost completely ionised in dilute solution, we have [NHi-] 
considerably increased, but as the product [NHi-J x [OH'] must 
remain constant, it follows that [OH'] must be correspondingly 
decreased. 

This reasoning is exactly analogous to that of the case of 
acetic acid and sodium acetate, discussed on p. 658, except that 
in this latter case the [H-] is considerably decreased by 
the addition of sodium acetate to the weak acetic acid. 
Michaelis has calculated the [OH'] of various mixtures of am- 
monium hydroxide and ammonium chloride, which are given in 
the following table. At 18° C. however 

[H-] X [OH'] =0-64 X IO-l^ 

so that knowing [OH'] it is possible to calculate [H-]. For 
curve plotting the symbol P^, which is — log^^ [H-], is used, 
therefore both [H-] and Pjj values are appended to Michaelis' 
figures. 



CAUSTIC ALKALINITY OF LIME LIQUORS 669 

Table 

Calculated figures by Michaelis showing [OH'] of various 
mixtures of ammonium hydroxide and ammonium chloride, 
and also the corresponding [H-] and Ph values. 



"O^J^ 




Percent. 


Per cent. 








Katiu, 
NH4OH : NH4CI. 


Njio 
NH4OH. 


iV/io 
NH4CI. 


[OH']. 


[H-]. 


Ph. 


32: 


I 


967 


3-3 


6-4x10-* 


IX 10-11 


II-O 


16: 


I 


94-1 


5-9 


3-2XI0-* 


2X10-11 


10-7 


8: 


I 


88-9 


ii-i 


1-6x10-* 


4x10-11 


10-4 


4: 


I 


8o-o 


20-0 


8 X 10-5 


8x10-11 


lO-I 


2 : 


; I 


667 


33-3 


^4X10-5 


1-6 X 10-^" 


9-8 


I : 


; I 


50-0 


50-0 


2 X IQ-^ 


3-2X10-1" 


9-5 


I : 


: 2 


33-3 


667 


I X 10-5 


6-4 X lO-i" 


9-2 


I : 


: 4 


20-0 


8o-o 


5 X io-« 


1-3x10-^ 


8-9 


I : 


: 8 


II-I 


88-9 


2-5x10-^ 


2-6X10-^ 


8-6 


I : 


:i6 


5-9 


94-1 


1-2X10-^ 


5-2x10-^ 


8-3 


I 


:32 


3-3 


967 


6x10-' 


I X 10-^ 


8-0 



It must be pointed out that dilution has little influence on the 
(OH') of mixtures of ammonium hydroxide and ammonium 

chloride, as it is really the ratio — -— ^— — that is the deciding 
•^ [NH4-] 

factor, as will be seen by re-writing equation (i) in the form 
rOH'l Kx[NH,O H] 

Thus from the table and curve it follows that a mixture of equal 
parts of ammonium hydroxide and ammonium chloride has 

[0H']=2XI0-^ 

corresponding to a Pg value of 9-5. 

Now, consider what happens when a solution of ammonia is 
titrated with standard acid, using phenolphthalein as indicator. 
Phenolphthalein has a range of colour change from Pjj=8-3 
(colourless) to Ph=io-o (deep red). Thus at Pji=8-3 ammonia 
is 94 per cent, neutrahsed, whereas at Pij = io-o it is only 24 per 
cent, neutralised. 

Not only have we to consider the ammonia, however, but 
Stiasny's results seem to suggest that the presence of calcium 
salts might have a profound influence on the [OH'], owing to the 
formation of complex calcium-ammonia ions, but experiments 
carried out on the following lines proved conclusively that cal- 



670 PRINCIPLES OF LEATHER MANUFACTURE 



cium salts do not alter the [OH'] of mixtures of ammonia and 
ammonium chloride to any appreciable extent. 

Various mixtures of AT/io ammonia and AT/io ammonium 
chloride were made up and the Pjj determined exactly as in the 
manner recently described for the determination of acidity in 
tan liquors {loc. cit.), except that, of course, a different indicator 







to 

"J 


UO bo 

% HH^OH. > 


eo 


so 


Fig. 130. 


20 



1' 



(phenolphthalein) and different standards were used. These 
standards were made up according to the instructions given by 
Cole. The actual values of Pg obtained were found to agree 
very well with the calculated figures of Michaelis. 

The P„ values of various mixtures of ammonia and ammonium 
chloride to which varying quantities of iV/5 calcium chloride 
or AT/io sodium chloride had been added were determined as 
shown in the table on next page. 

From these results it will be obvious that calcium salts do not 
cause any appreciable differences in the P^, and therefore in 



CAUSTIC ALKALINITY OF LIME LIQUORS 671 

Mixture consisting of 

c.c. Njio c.c. iV/io c.c. distilled c.c. N/io c.c. N/$ Ph 

NH4OH. NH4CI. water. NaCl. CaClg. observed. 

5 5 10 .. .. 9-45 

5 5 •• •• 10 9-45 

5 5 •• 10 .. 9-45 

3-9 6-1 10 .. .. 9-3 

3'9 6-1 .. .. 10 9-3 

[OH'], so that on titrating a lime liquor with N/10 HCl the 
calcium or sodium chlorides so formed will not affect the [OH'] 
of the ammonium hydroxide. 

The effect of hydrosulphidesj;must now be considered. Sodium 
sulphide is hydrolytically dissociated in solution as follows : 

NagS + HgO-^NaOH + NaSH. 

The caustic soda of course forms part of the caustic alkalinity, 
but sodium sulphydrate is also hydrolysed to some extent in 
aqueous solution. 

Walker found experimentally that N/10 NaSH was hydrolysed 
to the extent of 0-14 per cent. : 

NaSH + HgO-^NaOH + HgS, 

whence [OH'] = -ooi4Xo-i 

= •00014, or 1-4x10-* grm.-mols. per liter. 

Knowing the dissociation constant of HgS, which is a very weak 
acid, it is possible to calculate the [OH'] of N/10 NaSH. 

We have NaSH + H20=NaOH+H2S. 

If, therefore, we have i mol. of NaSH in " v" liters of water, 
and of this a fraction " a; " is hydrolysed, it has been shown that 



hx Kw 
x= I — - — , 
ka 

where Ki& is the dissociation constant of water (at 25° C. 
Kw=i-2Xio-^^) and ka is the dissociation constant of HgS 
(ka==^^-yxio-^ Sit 25° C), so by substituting these values in 
the above equation and putting 

w=io (for decinormal NaSH), 

, /lOX 1-2X10-^* o 

we have x= / = 1-45x10-^, 

\J 57x10-8 

or the concentration of NaOH, and consequently of the OH' ion, 
is 1-45x10-3 mols. in 10 liters, or [OH'] is 1-45x10-* normal, a 
result that agrees with Walker's experimental figures. 



672 PRINCIPLES OF LEATHER MANUFACTURE 

Now, in practice lime liquors are seldom used with a higher 
concentration of sulphide than corresponds to N/20 in sodium 
sulphydrate. 

If we substitute y=2o in the above equation, we then have 



20X1-2X10-1^ „ 

=2-05 X 10-^ 

57x10-8 

or [OHi] =2-05 X 10-3 X 0-05 

= 1-025 X 10-* normal, 

which at 25° C. corresponds to [H-] of 1-15 xio-^", or a Pjj 
value of 9-93, i.e. practically Pg^io-o. 

From the above it is clear that if a hme liquor be titrated to a 
point corresponding to Pjj=io-o the sodium and calcium hy- 
droxides will be practically completely neutralised, the ammonia 
will be neutrahsed to the extent of 24 per cent, (see curve), but 
the sulphydrate will not have been affected. Thus the titration 
will be a measure of a + b +o-24d. It is worthy of note that in all 
titrations to Ph = io-o no smell of escaping HgS was detected, 
but if the titrations were carried beyond this point the smell of 
HgS became quite distinct. 

Hence by estimating the ammonia, preferably by distillation 
in vactw, as described by Thompson and Suzuki, the value of 
a + h (the caustic alkalinity) may be obtained from two deter- 
minations instead of four, as described in our earlier paper. 

The choice of indicator in order to titrate to a P^ value of lo-o 
is important. Phenolphthalein has a range of colour change 
from 8-3 to lo-o, and at the latter value it is coloured deep red. 
As a general rule an indicator is not suitable near the limits of 
the colour change, so that the most suitable indicator for our 
purpose appears to be thymolphthalein, which changes from 
colourless to blue over the range of Pjj 9-3 to 10-5. Thus at 
Ph=io-o we are near the middle point of the colour change. 

The titration to a value of Pij=io-o was carried out in the 
comparator previously described {loc. cit.), using boiling tubes 
(6 inches by i inch) of the same internal diameter as shown 
in the diagram (p. 673). 

NJTO HCl was run slowly into tube Bg, stirring continuously, 
and when the match was nearly complete distilled water added 
until the total volume in Bg was nearly 50 c.c. (It is convenient 
to scratch a mark on the boiling tubes corresponding to a capacity 
of 50 c.c.) The titration was continued until the colour seen 
through the tubes A^ and Ag matched that seen through Bj and Bg. 

It was noted that the blue colour of the thymolphthalein 



CAUSTIC ALKALINITY OF LIME LIQUORS 673 

indicator faded after some time, so it is better to make up tube A2 
each time that a titration is being carried out. 

LIGHT 



25 c.c. Kme Hquor 

+25 c.c. distilled water. 



50 c.c. solution of standard 
alkalinity Ph=io-o. 

10 drops of thymolphtha- 
lein solution. 

Ground glass screen 



50 c.c. distilled water. 



B2 

25 c.c. lime liquor 
+ 10 drops thymoJphtha- 
lein solution. 



EYE 



As the standard solution of Pjj = io-o, a mixture of 76 per cent. 
N/10 NH4OH and 24 per cent. N/ip NH4CI was used. A hter 
of solution of this composition may be made up and kept in a 
stoppered bottle, the thymolphthalein being added to 50 c.c. 
as required. 

Determinations of the caustic alkalinities of four lime liquors 
were made, and these results compared with results obtained by 
the method previously described. All titrations are given in 
terms of c.c. AT/io acid or alkali per 25 c.c. of filtered lime liquor. 

No. of Lime Liquor .1 2 3 4 

[ Titration to Ph=io-o . io-2 16-45 i6-8 17-05 
Method {a + b +0-246..) 

described Ammonia {d) . .0-1 12- 1 4-8 7-0 

in ] o-24d -02 2-9 1-15 1-7 

this 
paper. 



Method 

of 

Atkin 

and 

Palmer. 



.-. a + b . . io-i8 
(caustic alkalinity) 


13-55 


15-65 


15-35 


ist H . CHO titration . 9-1 

{a + b+c — amino acids) 
2nd H . CHO titration . i-i 


13-3 

12-6 


19-9 
5-0 


18-9 
7-35 


(^ + amino acids) 
Ammonia (d) . .0-1 


12-1 


4-8 


7-0 


Sulphide (c) . . .0-0 


0-6 


4-4 


3-7 



-. a + b 
(caustic alkalinity) 



lO-I 



13-2 15-7 15-55 



It will thus be seen that the agreement between the two methods 
is quite satisfactory. 

43 



INDEX 



Abderhalden, 136; (and Fischer), 

137- 
Abies, 282. 

Absolute zero of temperature, 85. 
Absorption isotherm, 94. 
Abt, 36, 38. 
Acacia, 329. 
Acid colours, 488. 

— deliming, 355. 

gelatin equilibrium, 626. 

— value of oils, 433. 
Acidity in tan liquors, 656. 

— of liquors, 370. 

— of tan liquors, 656. 

— of tanning materials, 666. 
Acids, action of, 19. 

— alkalies, and salts, action on 

gelatin, 115. 

— amino-, 127. 

— organic, 127. 

— strength of, 99. 
Acree, 661. 
Acrolein, 449. 

Action of acids on gelatin, 115. 
Adipose layer, 61 . 
Adsorption, 93. 
Aeschern, 181. 
..^thalium septicum, 10. 
African oak, 293. 
Ageing, 247, 252. 
Ailantus glandulosa, 311. 
Air filters, 530. 

lift for liquors, 554. 

Albizzia, 335. 

Albumins, 131, 154. 

Albumoses, 132. 

Alcohol and gelatin, 585, 587. 

Alcoholic fermentation, 16. 

Alcohol, osmotic pressure, 115. 

— precipitation by, 115. 
Aldehydes, 461. 
Alder, 286. 

Aleppo pine, '2 83. 

Algae, 27. 

Algarobilla, 328. 

Alkaline carbonates in liming, 184. 

— hydrolysis of proteins, 135. 
Alkalinity of limes, 667. 
Allen, 446. 

Alleyways, 550. 
Alnus, 286. 



Alumina in water, 79. 

— salts, 241. 

- — sulphate, 209. 

Aluminium sulphate, 241. 

" Aluminoferric," 564. 

Alums, 241. 

Alum tannage, 240. 

American gallon, 581. 

Amines, 221. 

Amino-acids, 127. 

• — action of formaldehyde, 144. 

— proteins, 135. 
Ammonia, 16. 

— in unwooling, 33, 166. 
Ammonium chloride, 209. 

— sulphate, 209. 

— ■ — for pickling, 576. 
Amoeba, 10. 
Amphoteric acids, 128. 

— proteins, no. 
Anacardiacese, 306. 

Anaerobic and aerobic bacteria, 567. 
Analysis of oils and fats, 431 
Andreasch, 309. 
Angica bark, 334. 
Aniline dyes, 487. 
Anode, 97. 
Anogeissus, 322. 
Anthrax, 26 

— bacterium, 17. 
Anticalcium, 29. 
Antiseptics, 21. 

Apocynacese, 318. / 

Arbutus, 319. 

Archbutt and Deeley, 68. 
Arctostaphylos, 319. 
Areca, 284. 
Arisz, 113. 
Arrector pili, 55. 
Arsenic, 27. 

— " cures," 42. 

— red, 185, 189. 

— sulphide, 27. 
Arsenious acid, 27. 
Aspidospermum, 307, 318. 
Atkin, 656. 

— J- 667. 

— W. B., 667. 
Atoms, 84. 
Attfield, 41. 
Automatic drier, 534. 



674 



INDEX 



67- 



Avaram bark, 329. 
Avogadro's law, 86. 
Azo-dyes, 505. 

Bablah pods, 330. 
Babool pods, 214. 
Babul, 329. 
Bacteria, 14, 15. 
Bacterial filters, 568. 

— poisons, 18. 

— purification, 565. 

of sewage, 567. 

Bacterium furfuris, 214. 
Badamier bark, 321. 
Bag-tanning, 372. 
Bain, 540. 

Bakau bark, 323. 

Balancing high - speed machines, 

383. 
Baldwin, 656. 
Ball valves, 551. 
Bandknife machine, 463. 
Banksia, 303. 
Barbatimao bark, 335. 
Barbed-wire scratches, 43. 
Barberry juice, 275. 
Bark breakers, 388. 

— mills, placing, 546. 

— structure, 279. 
Barytes, 468. 

Basic colours, 488, 504. 

— salts, 24-2. 

Basicity of chrome liquors, 269. 

— of solutions, 266. 
Bast cells, 281. 
Bastin, 280. 

Bate, C. T., 29. 
Bate-shavings, 560. 
Bating, 8, 201, 218. 
Bauhinia, 335. 
Bearberry, 319. 
Bechhold, iii. 
Becker, 36, 37, 38. 
Bedda nuts, 321. 
Beeswax, 451. 
Bell-mills, 381. 

mouthed ducts, 529. 

Belt-driven pumps, 552. 

Belts, choice and treatment, 544. 

Benedicenti, 146. 

Bennett, 668. 

Benzoic acid, 29. 

Berkefeld candles, 352. 

Bernardin, 278. 

Betel nut, 284. 

Betula, 287. 

— alba, 452. 
— lenta, 453. 
Bichromate of potash, 377. 

— reduced with SOg, 572. 



Bichromates in dyeing, 506. 

— or dichromates, 255. 
Biernacki, 21. 
Biggin, 348. 
Bilberry, 319. 

Birch, 287. 

— tar oil, 31. 
Bisulphate of soda, 204. 
Bisulphites, 25. 
Biuret test, 140. 
Black birch, oil of, 31. 

— dyeing, 506. 
Blackman fan, 526. 
Bleaching, 360. 

— extracts, 352. 

— powder tannage, 575. 

Blair, Campbell & M'Lean, Ltd., 

408. 
Blistering of dried hides, 42. 
Blockey, 268. 

— chrome liquors, 268. 
Blood-corpuscles, 10, 12. 

— white, 60. 

" Blood crystals," 403. 
Bloom, 341. 

— of oils, 444. 
Blosse, 50. 
Bluebacking, 273. 
Boakes, Ltd., 26. 
Bogue, 150. 
Boiled oils, 443, 479. 
Boiler compositions, 77. 

— house, 541. 
Boiling point, 87, 513. 
Borax, 205. 

— and other borates, 26. 
Borgman, 231. 

Boric acid and phenol for deliming, 
212. 

— (boracic) acid, 26, 205. 
Borke, 280. 

Bot flies, 43. 

Bottger, 187. 

Bottlenose-oil, 450. , 

Box, 523. 

Boyle's law, 85. 

Brabium, 304. 

Bran drench, 18, 214. 

Brands, 43. 

Brazilwood, 329, 505. 

Breaking stress of leather, 545. 

Brick pits, 548. 

Bromine as tanning agent, 575. 

Bronzing, 488, 497. 

Brough, 570. 

Brown and Millar, 144. 

Brownian motion, 107. 

Browns, 510. 

Brugniera, 323, 

Brumwell, 318, 336. 



676 PRINCIPLES OF LEATHER MANUFACTURE 



Brunner, 41. 

Brusca, 319. 

B. T. U., 514. 

Buffalo method of liming, 177. 

— pits, 549- 

" Buffers," 370. 

Buff-leather, 457. 

Buildings, arrangement of, 539. 

" Burning in," 469. 

Burns and Hull, 213. 

Burton and Hey, 255. 

Butea, 326. 

Biitschli, 112, 588, 614. 

Butyric acid, 208. 

Byrsonima, 305. 

Bystron, 277. 



Csesalpinia, 327. 
Calcium butyrate, 18. 
■ — lactate, 18. 

— sulphydrate, 187. 
Calf- kid, 245. 
Callitris, 284. 
Calorie, 514. 
Cambium, 279. 
Campbell, 207. 
Camphor, 31. 
Canaigre, 300. 
Candy, 565- 

Cane sugar, 16. 
Caparossa bark, 324. 
Cape sumach, 304. 
Capel fan, 529. 
Carbolic acid, 21, 27. 
Carbolineum, 28. 
Carbon disulphide, 30. 
Carbonic acid, 16. 

in water, 80. 

Carissa, 318. 
Carmichael, 573. 
Carnaiiba wax, 451. 
Carpenter, Prof. G. H., 44. 
Carr, 382. 
Cascalote, 327. 
Casein, 131, 154. 
Cassia, 329. 

— bark, 498. 
Castania, 287. 

Casting or pitching leaches, 398. 

Castor oil, 441. 

Casuarinae, 285. 

Catalysis, 15. 

Catechol tans, 339. 

Catechu, 330. 

Caustic soda, 21. 

Cavallin, 256. 

Ceanothus, 326. 

Cebil bark, 334. 

Celavina, 329. 



Cell, the living, 10. 

Cells, multiplication of, 11. 

Cellulose, 12. 

— acetate, 486. 

Celsius thermometer, 581. 
Cement-substance, 60. 
Centigrade-Fahrenheit table, 581. 

— thermometer, 581. 
Centrifugal fans, 529. 

— pumps, 552. 
Ceresin, 455. 
Ceriops, 323. 

Chain conveyors, 547. 

Charges of colloids, no. 

Chenalier evaporator, 516. 

Chestnut, 287. 

Chilco bark, 324. 

Chlorine as tanning agent, 575. 

— in water, 80. 

Chloroform, action on ferments, 

17- 
Chrome calf-kid, 249. 

— combinations, 377. 
tannages, 271. 

— liquors, basicity, 269. 

Blockey's, 268. 

concentrated, 267. 

reduced by SO2, 267. 

— salt in, 269. 

— sole leather, 270. 

— tanning, 9. 
Chromium, salts of, 254. 
Chromoproteins, 131. 
Churco bark, 319. 
Clackvalves, 551. 
Claflin, 207, 656. 
Clark, 67, 68. 
Clearing, 503. 

— leathers, 497. 
Cleistanthus, 305. 
Climbing film evaporator, 408. 
Cloves, oil of, 31. 

Coal per horse-power, 515. 
tar blacks, 493. 

— • — odours, 487. 

Coccus bacteria in salt-stains, 37. 

Cockle, 46. 

Cocoloba, 303. 

Coconut palm, 285. 

Cocos, 285. 

Cod oil, 445. 

Cole, 137. 

Cole, S. W., 155. 

Cole and Onslow, 661. 

Collagen, 146. 

— equivalent weight of, 123. 
Collin and Benoist, 26. 
Colloid state, 107. 
Colloids, 94. 

— emulsion, no. 



INDEX 



677 



Colloids, organic, no. 
Colorimetric method, 103. 
Colour bases, 488. 

— measurement, 353. 

— mixing, 508. 

— reactions of proteins, 140. 
Colpoon, 304. 

— compressa, 311. 
Colt, 47. 

Combination-tannages, 253, 375. 
Combretaceae, 320. 
Commercial facilities, 539. 
Comparator, 103, 578, 661, 

663. 
Compressed air, 553. 
Condensed water, 533. 
Cone-mills, 381. 
Connective tissue, 48. 

fibres, 58. 

Construction of tanneries, 538. 
" Contact beds," 568. 
Conveyors, 389, 391, 547. 
Cooling of evaporation, 519. 
Copal varnish, 481. 
Copernica cerifera, 451. 
Copper, lead, in water, 80. 

— sulphate, 27. 
Coriaria, 316. 

— myrtifolia, 311. 
Coriariaceae, 316: 
Coriin, 60. 

Corium, 48, 49, 50, 57, 61. 

Cork, 280. 

• — • oak, 293. 

Corneum, 51. 

Cortegia rossa, 283. 

Cost of acids for deliming, 209. 

Cottonseed oil, 444. 

Couperus, 316. 

Couratari, 324. 

Creasote, 28. 

Creolin, 28. 

Cresols, 28. 

Cresotinic acid, 212. 

Critical state, 87. 

Crossostylis, 323. 

Crown leather, 459. 

Crystalline form, 95. 

Crystallisation of proteins, 141. 

Crystalloids, 94. 

Cuir en tripe, 50. 

Cupania, 325. 

Curing skins by alum, 244. 

Currying, 463. 

Curtidor bark, 319. 

Curupi bark, 334. 

Curve of variable e, 634. 

Cutch, 330. 

black, 507. 

Cutis, 48. 



Dacca kips, 39, 40. 

Dakin, 135, 136, 151. 

Dalton's law, 85. 

Damaged hides, 42. 

Danish glove leather, 375, 377. 

Daphne, 304. 

Daphnoidse, 304. 

Dasselplage, 43. 

Davis, 481. 

Davy, 348. 

Dead-fat, 47. 

Decimal system, 580. 

Decolorising of extracts, 403, 404. 

Defects in dyeing, 502. 

Degras, 448, 459. 

former, 450. 

Dehydrating effect of salt, 22, 234. 
Dehydration by alcohol, 114. 
Dekker, 278, 347. 
Deliming with acids, 202. 
De Lof, 278, 290. 
Denaturised salt, 23. 
Dennis, 264. 
Depickling, 237. 

— with " hypo," 238. 
Depilation, 7. 

— for glove kid, 250. 
Derma, 48, 50. 
Dermestes vulpinus, 42. 
Dialysis, in. 
Diazotising, 492. 
Diffusion, 95, 363, 373. 
Dilute acids and gelatin, 583. 
Dilution formula, 99. 
Dipolarising effect, 113. 
Dipterocarpus, 335. 
Direct-acting pumps, 551. 
Disinfectants, 21. 
Disintegrators, 382. 
Dispersity, 578. 

Disposal of sewage, 538. 
Dissociation-constant for water, 660. 
Distilled stearine, 440. 

— wool grease, 439. 
Distribution between solvents, 93. 

— of air, 537. 
Divi-divi, 327. 

Doerr and Reinhart, 485. 
Dog-dung, 225. 
Dongola imitations, 379. 

— leather, 375, 376- 
Donnan, 582. 
Donnan and Harris, 118. 
Drain-cleaning rods, 550. 
Drawn grain, 361. 
Dreher, 504. 

" Dreikronenthran," 447. 
Drenching, 8.. 201, 214. 

— fermentation in, 19. 
Drepanocarpus, 327. 



678 PRINCIPLES OF LEATHER MANUFACTURE 



Dressing leathers, 369. 
Driers for oils, 443. 
Drum dyeing, 500. 

— stuffing, 467. 

— for washing, 165. 

Dry hides, soaking of, 159. 
Drying fleshings, 559. 

— hides, 41. 

— of leather, 516. 

— of oils, 428, 430. 

— of stuffed leather, 466. 

— power of air, 518. 

— sole leather, 365. 

— with heat, 521. 

Dry-salted hides, soaking of, 159. 
Dubbing, 465. 
Dyebaths, re-using, 502. 
Dyeing alumed leathers, 495, 496. 

— chrome leather, 294. 

— defects, 497. 

— oil-leathers, 496. 

— in two trays, 501. 
Dye manufacturers, 640. 
Dyes and dyeing, 487. 

— for leather, 640. 
Dye trials, 511. 

Earle, 483. 

Earp, W. R., 190. 

East India sheep and goat, 371. 

Eau de Javelle, 575. 

Eberle, 208, 224, 265. 

" Economisers," 525. 

Effront, 208. 

Egg-albumin, 131. 

Einbrennen, 469. 

Eitner, 23, 27, 28, 81, 82, 159, 162, 
179, 216, 259, 261, 262, 263, 
266, 268, 269, 272, 273, 288, 
376, 445, 446, 449, 460, 470. 

Elastic fibres, 58, 60, 63. 

Elasticity of puered skin, 232. 

Elastin, 153. 

Electric charges of sols, 108. 

— currents, 85. 

— driving, 543. 

— osmose, 109. 

— tanning, 109. 
Electrolysis, 97. 
Electrometric method, 103, 104, 

578. 
Electrons, 84. - 

Eleidin, 51. 
Elephantorrhiza, 335. 
Ellagic acid, 341. 
Emulsification, 465. 
Emulsion colloids, no. 
Emulsions, 91, 472. 
Enamelled leather, 475. 
Engine, position of, 542. 



Enterokinase, 223. 
Enterolobium, 335. 
Enzyme bates, 578. 

— hydrolysis, 136. 
Enzymes, 15, 16, 126, 136, 200. 

— from puer, 221. 
Epidermis, 13, 19, 48, 49, 50. 
Epithelium, 48. 

— cells, 13. 
Equilibria, 87. 

— ionisation, loi. 
Erector pili, 53, 54, 55. 
Ericaceas, 319. 

Ernst and Zwenger, 341. 
Erodin, 222. 
Essential oils, 31, 452. 
Eucalyptus, 324. 
Eucoupia, 336. 
Eudermin, 28. 
Eugenia, 325. 
Euphorbiaceae, 305. 
Evaporation, 512. 

— heat of, 87. 

— in open pans, 515. 

— in vacuo, 515. 

— of extracts, 405. 

— theory of, 512. 
Excelsior mill, 382. 
Exocarpus, 304. 
Extensions, 541. 

Extraction of tanning materials, 

392. 
Extractors for sugar-beet, 402. 

Fading by light, 498. 
Fahrion, 425, 575. 
Fan-drying, 520. 
Farad, 97. 
Faraday, 479. 
Fat-cells, 22, 61. 

distilling, 428. 

glands, 49. 

liquoring, 273, 378, 471. 

recovery, 558. 

— solvents, 428. 
Fats and oils, 425. 

— in currying, 463. 
Fatty acids, 425. 
Feminella, 308. 
Fer Bravais, 108. 
Fermentation, 15. 
Ferments, unorganised, 15. 
Ferric oxide, 22. 

— ■ in place of Prussian blue, 485 . 
Filao bark, 285. 
Filter-pressing grease, 559. 
Filters for water, 78. 
Filtration of sewage, 565. 
Finishing sole leather, 365. 
Fire risk, 388, 540, 546. 



INDEX 



679 



Fischer, E., 135, 340, 639. 

— and Abderhalden, 137. 
Fish tallow, 448. 
Fixing tannin, 503. 

" Flaming," 499. 
Flaying of hides, 42. 
Fleshing, 7, 193. 

— machines, 195. 
Fleshings, 556. 

— drying, 559. 
Flooring, 553. 
Flower, G. W., 171. 
Fluorescence of oils, 454. 
Fluorides, 27. 
Flywheel pumps, 551. 
Fly wing skiver, 62. 
Fog, 518. 

" Foots," 431. 
Formaldehyde, 30, 343. 

— action on amino-acids and 

proteins, 144. 

— tannage, 576. 
Formic acid, 208. 

— for pickling, 236. 
Formol, formalin, 30. 
Fractional precipitation of proteins, 

112. 
Fraymouth and Pilgrim, 318, 
French calf, 210. 
Freudenberg, 340. 
Freundlich, 614. 
Frizing, 457. 
Fuchsia, 319, 324. 
Fungi, 15. 
Fur-dressing, 457. 
Fusanus, 304. 

Gaillet-Huet, 71. 
Gallic acid, 338, 574. 
Gallotannin, 339. 
Galls, 297. 
Gambler, 316. 

— pods, 329. 
Garcinea, 335. 
Gas-constant, 86. 

— engines, 543. 

— equation, 86. 
Gases, 85. 
Gas-lime, 187. 

" Gathering limes," 33. 
Gaultheria procumbens, 453. 
Gay-Lussac's law, 85. 
Gelatin, 149. 
— , acetic acid and salt, 61 1 . 

— amphoteric character of, 116. 

— and acids, 586, 588, 594. 

— and HCl., 591. 
• — and salt, 600. 

— and sodium acetate, 609. 
formate, 608. 



Gelatin and sodium^sulphate,'"6o6. 

— and weak acids, 123. 

— coagulation by reagents, 150. 

— combining equivalent, 619, 630. 

— composition of, 136. 

— curves, 592. 
equation, 122. 

— elastic cohesion of, 119. 
— • equivalent weight of, 123. 

— experiments on, 120, 121. 

— HCl and KCl, 612. 
NagSOi, 610. 

— hydrolysis, 152. 

— in alcohol, 585, 587. 

— in water, 584. 

— ion, 117. 

— ionisation-constant, 621. 

— jelly, 113. 

— optical activity, 150. 

— purity of, 149. 

— reactions of, 151. 

— swelling curves, 121, 122. 

— swelling equilibrium of, 116, 

117. 

tannin reaction, 151. 

Gelatoses, 132. 
Gels, 108. 
Gingeli oil, 444. 
Glaser, Dr Hans, 43. 
Glasig, 215. 
Glazing, 510. 

— chrome skins, 275. 
Gliadins, 131. 
Globig, 17. 
Globulins, 131. 
Glove-kid, 250. 
Glover, A., 73. 
Glucoproteins, 131. 
Glucose, 16, 468. 

— bates, 228. 

— fermentation of, 13, 15. 

stuff, 556. 

Glue-boiling, 556. 

drying, 558. 

fats, 437. 

Glutamic acid, 127. 

Glycerides, 425. 

Glycine or glycocoll, 127. 

Glyoxylic reaction, 140. 

Goad-marks, 43. 

Golden tan, 284. 

Gold-sols, 107. 

Gool-i-pista, 307. 

Grain, 61. 

Graining leather, 369. 

Grain layer, 50, 59. 

Granataceae, 326. 

Grape sugar, 16. 

Grasser, 343. 

Grease extraction, 556. 



68o P-RINCIPLES OF LEATHER MANUFACTURE 



Green leather, 377. 
Green oak, 292. 
Greens, sage and olive, 510. 
" Greenstiffness," 179. 
Grevillia, 303. 
Griessmayer, 341. 
Grinding samples, 347. 

— tanning materials, 380. 
Grounding, 244. 

" Gumming," 20. 
Gum tragacanth, 508. 
Gunnera, 324. 
Gunneraceas, 324. 

Hsematin, 56. 
Hasmoglobin, 131. 
Hair, 555. 

muscle, 49, 53, 54, 55. 

pores, 61. 

sheaths, 55. 

— structure of, 52 
Hairs, 49. 

Halogens, action on proteins, 146. 

Hampshire, B., 167. 

Handlers, 358. 

Hard greases in stuffing, 468. 

— water, effects, 74. 
Hardness, degrees of, 67. 

— effect on hides, 81. 

— of water, 66, 67. 

— permanent, 76. 

— temporary, 67. 
Harrison, 310, 569. 
Hauff, 212. 
Hausmann, 143. 
HCl and gelatin, 591. 
Heal and Procter, 257. 

Heat, effect on softening hides, 159. 

— given by steampipes, 524. 

— loss through roof, 523. 
walls, 523. 

— of evaporation, 87, 515. 

— of thawing ice, 515. 

— quantity of, 514. 

— required to warm air, 520. 
Heating leaches, 400. 

— of chamois leather, 458. 

— of leather, 20. 
Hebner, 67. 
Hjinzerling, 256. 
Helvetia leather, 459. 
Hemlock bark, 282. 
Hennig, 480. 
Henry, T., 68. 

Hide a colloid jelly, 116. 

fibre acid and basic, 490. 

-^ -mill, 163, 164. 

— powder, 579. 

• process, 349. 

Hides, marking weight, 32. 



Hides, South American, 42. 
High-speed machinery, balancing, 

545- 
Historical notice, 1. 
Hofmeister, 129, 147, 624, 625. 
Holbrook system, 395. 
Holden fat, 439. 
Hollander, 190. 
Hooke's law, 634. 

of elasticity, 119. 

Hooper, 278, 323. 
Hopkins and Cole, 140. 

— and Pinkus, 141. 
Horns, 560. 
Horse-fat, 438. 
power, 515. 

Hot-air stuffing drum, 467. 

■ — -water heating, 533. 

Hughes, 291. 

Humin nitrogen, 143. 

Hummel, 257. 

Hunt, 244. 

Hurwitz, Meyer and Osterberg, 661. 

Hyaline, 56, 491. 

Hydric sodic sulphite, 25. 

— sodium sulphate, 22. 
Hydrion-concentration, 103, 106. 
Hydrogenation, 432. 
Hydrolysis, alkaline, of proteins, 135. 

— by enzymes, 136. 

constant, 102. 

equation, 102. 

for gelatin, 618. 

— of proteins by acid, 134. 

— of proteins, 128, 132. 
Hydroquinone, 573. 
Hyphse, 14. 

Hypo, uses of, 238. 
Hypoderma bovis, 43, 44, 45. 

— lineata, 44. 

Immiscible solvents, 93. 
Indian cure of kips, 39. 

— kips, 160. 
Indicating engines, 542. 
Indicators, 103. 

— table of, 662. 
Inga, 334. 
Inks, 494. 

" Insolubles," 351. 
Interfaces, liquid, 90. 
Internal pressure, 88. 
Introductory sketch of leather 

manufactures, 3, 7. 
Invertase, 16. 
Involuntary muscle, 55. 
Iodine as tanning agent, 575. 

— value, 428. 

of oils, 434. 

lonisation, 97, 



INDEX 



68 1 



lonisation constants, 99. 

— of water, loi, 106. 

— of weak acids, 657. 
Iron-alum, 276. 

blacks, 491, 493. 

— blues and browns, 492. 

— in blood, 38. 

— in water, 79. 

— liquor, 506. 

— pyrolignite, 506. 

— salts of, 254. 
solutions, 506. 

— tannages, 275. 
Isoelectric point, iii, 118, 370. 
of collagen, 201 . 

of gelatin, 663. 

Itcha, 33. 
Izal, 28. 

Jamrosa bark, 321. 
Japan for leathers, 444. 

— preparation of, 478. 
Japanese leather, 461. 
Japanned leathers, 475. 
Japanning, American, 481. 

— leathers suitable, 476. 
Japans, application, 477. 

— currying, 476. 
■— drying, 477. 
Jellies, 112. 
Jettmar, 263, 277. 
Jeyes' fluid, 28. 
Junghans, 485. 
Juniperus, 284. 

Kahua bark, 321. 

Karunda, 318. 

Kaspine leather, 459. 

Kataphoresis, 109. 

Kath, 331. 

Kathode, 97. 

Kathreiner, F., 56, 252, 346. 

Kent, 375, 376, 378. 

Keratins, 134, 153. 

Kermes oak, 294. 

Kestner evaporator, 408. 

Kilogrammeters, 515. 

Kino, 326. 

Kips, Indian, 160. 

Kiri-toa-toa, 284. 

Kittsubstang, 60. 

Klemm, 460. 

Knapp, 115, 243, 256, 264, 276, 461, 

575- 
Knoppern, 298. 
Knotted tree, 304. 
Koch, 288. 
Koenig, 570. 
Koerner, 323. 
Kolliker, 51. 



Korner, 114. 
Krameria, 305. 
Klihne, 137. 
Kundt, 114. 
Kyrins, 132, 133. 

Lactic acid, 18, 207. 

— anhydride, 208. 

— fermentation, 18. 
Laguncularia, 322. 

Lamb, 274, 310, 487, 488, 498, 571, 

640. 
Lanoline, 439. 
Larix, 282. 
Laurus, 303. 
Law, 232. 
Layers, 359. 
Leach bottoms, 393. 

casting machine, 399. 

Leaches and pipes, 549. 
Leaching, 392. 

— batteries, 395. 
Lead bleach, 492. 
Leadwort, 304. 
Lead yellows, 492. 
Leaf-cuticles, 281. 
Leather-scrap, 570. 
Lecythidacese, 324. 
Lecythis, 324. 

Leidgen unhairing machine, 192. 

Lenticels, 281. 

Lepetit, Dollfus and Gausser, 342, 

Leucodendron, 304. 

Leucospermum, 304. 

Levites, 143. 

Lewkowitsch, 425, 446. 

Lieberkiihn's jelly, 132. 

Liebig, 126. 

Lietzmann, 461. 

Lime, 21. 

— action of on hides, 173. 

— analysis, 171. 

— " available," 172. 

— liquors, 134. 
alkalinity, 667.- 

— slaking, 169. . 

— solubility, 170. 
pits, 548. 

water method, 665. 

water test, 357. 

Limes, bacteria in, 178. 

Liming, 168. 

• — fermentation in, 19. 

— in suspension, 174. 
Linolenic acid, 430. 
Linseed mucilage, 508. 

— oil, 442. 
Liquid state, 86. 
Liquor tanks, 397, 551. 

— troughs, 399. 



682 PRINCIPLES OF LEATHER MANUFACTURE 



Liquor valves, 397, 550. 

Liquors, raising by compressed air, 

554- 
Litharge, 479. 
Lloyd, Miss D. J., 150. 
Loeb, III, 117, 182, 577, 583, 639. 
Logarithmic expression of numbers, 

105- 
Logwood, 329, 505. 

— blacks, 493. 
Lovibond tintometer, 353. 
Lowe, 341. 
Lowenthal, 348, 349. 
Loxopteryngium, 306. 
Lubricating oils, 547. 
Lubs, 664, 665. 
Ludwigia, 324. 
Liippo-Cramer, 577. 
Lymph-corpuscles, 10. 
Lysine, 127. 

Lysol, 28. 
Lythracese, 336. 

M'Bain, 656. 
M'Candlish, 270. 
Machires, position of, 542. 
Magnesia hardness, 68. 
Maiden, 332. 
Malignant pustule, 17. 
Mallet bark, 325. 
Malpighia, 305. 
Malpighiacese, 305. 
Manchester yellow, 499. 
Mangifera, 316. 
Mangosteen, 335. 
Mansfield Clark, 661, 665. 
" Marking off," 499. 
Marking weight of hides, 32. 
Maroons, 510. 
Marr, 534. 

— system of drying, 534. 
Marriott, R. H., 51, 52, 57, 59. 
Marsh gas, 16. 

— rosemary, 305. 
Marshall Ward, 279. 
Martin's yellow, 499. 
Mascolino, 308. 
Mass law, 99. 
Mather and Piatt, 70. 
Matter, 84. 

— states of, 85. 
Maynard, 161. 
Mehapore kips, 39, 40. 
Mellowness of liquors, 362. 
Melting ice, heat of, 88. 
Melting point of fats, 435. 

— of jellies, 114. 
Membrane equilibria, 582. 

potential, 118. 

Menhaden oil, 430. 



Mercuric chloride, 21, 26. 

— iodide, 26. 
Mercury lamps, 484. 
Metabisulphate of soda, 210. 
Metabisulphite of soda, 26, 267. 
Metals in dye- vats, 511. 
Metaprotein, 132. 

Methyl salicylate, 31, 287, 453. 
Metrical-British table, 580. 

— system, 580. 

Meunier, 237, 573, 574, 575, 576. 

Michaehs, 668, 669. 

Microscope, 65. 

Migratory cells, 60. 

Millon's reaction, 140. 

Mimosas, 332. 

Mimosese, 329. 

Mineral acids as antiseptics, 23. 

— oils and waxes, 453. 
Mixed colours, detection, 502. 
Moellon, 448, 459. 

Moeller, 223. 
Mohr's liter, 581. 
Moisture in leather, 519. 

— in stuffing, 465. 

— necessary to fermentation, 18. 
Molisch's reaction, 141. 
MoUerstein, 475. 

Moon knife, 244. 
Moos and Kutsis, 277. 
Mordants, 491. 
Mossop and Garland, 212. 
Moulos, 13, 15. 
Mountain ash, 326. 
'Mucins, 130, 131, 154. 
Mucous layer, 19. 
Mud in water, 78. 
Muir, J., 188. 
Multiple effects, 407, 515. 
Muscle, voluntary, 63. 
Muscular contraction, 625. 
Mutual precipitation of colloids, 

109. 
Myricaceae, 286. 
Myrobalans, 320, 305. 
Myrsine, 303. 
Myrtaceae, 324. 
Myrtus, 325. 

Nance, 572. 
- — process, 373. 

— tannage, 513. 
Nancite, 305. 
Naphthols, 29. 
Nauclea, 316. 
Neatsfoot oil, 438. 
Neb-neb pods, 330. 
Nematode worms, 167. 
Neradol, 343. 
Nervous ganglia, 61. 



INDEX 



683 



iSTesbitt, 211. 

Neutral salts for deliming, 209. 

weakening effect of, 100. 

Neutralisation, 473. 

— of chrome leather, 272. 
Neutrality of water, 106. 
Nihoul, Prof., 76. 80. 
Nitre-cake, 22. 
Nitrocellulose, 483. 
Nitrogen in proteins, 142. 
Non-drying liquid fatty acids, 429. 
Non-tans, tanning with, 574. 
Nucleolus, 1 1 . 

Nucleus, II. 

Oak bark, 289. 
Oaks, 288. 
Oakwood, 290. 
Octobromides, 430. 
Oil boiling, 443, 479. 

— engines, 543. 

— from grease, 559. 
leathers, 9. 

soluble colours, 488. 

— tannages, 457. 

Oils and fats, table of constants, 
. 436. 

— in currying, 463. 
" Oleine," 440. 
Oleo-stearine, 437. 
Olive oil, 440. 
Onagraceae, 324. 
Ooze calf, 375. 
Open-air drying, 519, 521. 
Optimum extraction temperature, 

tables, 414-424. 

— of swelling, 117. 

— temperature of extraction, 412. 
Ordoval, 344. 

Organic colloids, no. 
Ormerod, Miss E. A., 43. 
Orpiment, 189. 
Osmotic pressure, 95. 
Ostwald, 99, 122, 590, 618. 
Osyris, 304. 

Ovum, devolopment of, 48. 
Oxalates, 209. 
Oxalideae, 319. 
Oxalis, 319. 

Oxynaphthoic acid, 213. 
Ozokerit, 455. 

Paddle dyeing, 500. 

Paessler, 625. 

— and Appelius, 205. 

Paget, 583, 590. 

" Painting " for depilation, 33. 

" Pairing," 499. 

Palmse, 284. 

Palmer, 15, 226, 230, 412. 



Palmetto, 284. 
Pancreatic ferments, 64. 
Pancreatin, 220. 
Pancreol, 222. 
Panniculus adiposus, 61, 63. 

— carnosus, 63. 
Papilionacese, 326. 
Papillae, 61. 
Paraffin oils, 453. 

— wax, 454. 
Paraform, 31. 
Parenchym, 279. 
Parker, 212, 412, 548. 
Pars fasciculi, 50. 

— papillaris, 59, 60, 61. 
Partial pressures, 86. 
Partition-constant, 615. 
Pauli, W., 155, 624, 625, 639. 
Paullinia, 325. 

Payne, M., 72, 183, 211. 

— and Pullman, 576. 
Payne-Pullman liming process, ^ 183. 
Peijbled grain, 369. 
Peltophorium, 329. 

Penicillium glaucum, 13, 26. 
Pepsin, 17, 137, 219. 
Peptides, 129. 
Peptisation, 107. 
Peptones, 132, 133. 
Perching, 244. 
Perkin, 341, 487. 
Permutit, 73. 
Persea, 303. 
Petroleum oils, 453. 
Pfeiffer and Modelski, 131. 
PH, 660. 

— and POH, 106. 

— standard solutions, 664. 
Phenol, 27. 

Phenols, 338. 
Phlobaphenes, 342. 
Phloem, 279. 
Phosphate leathers, 577. 
Phosphates, 209. 

— in salt-stains, 37. 
Phosphoproteins, 131. 
Phyllanthus, 305. 
Phyllocladus, 284. 
Physical chemistry, 84. 
Picea, 282. 
Pickling, 234. 

— process, 583. 

— with bleaching powder, 237. 
formic acid, 236. 

potassium carbonate, 237. 

Pilgrim, 321. 

Pimento, oil of, 31. 

Pine barks, 282. 

Pinus, 283, 284. 

Pipes, arrangement of, 531. 



684 PRINCIPLES OF LEATHER MANUFACTURE 



Pipes near ceiling, 528. 
Piquiren, 217. 
Pistacia, 307. 

— lentiscus, 313. 
Pithecolobium, 335. 
Pits, construction of, 547. 
Plaster cures, 39, 40. 
Platanus, 280. 

Pleating," 499. 
Plimmer, 129, 135, 155. 
Plumbaginae, 304. 
Plumbago, 304. 
Pneumatic rolls, 192, 199. 
Podocarpus, 284. 
Poisons, bacterial, 18. 
Polygalaceae, 305. 
Polygenetic colours, 492. 
Polygonaceae, 300. 
Polygonum, 302. 
Polypeptides, 129, 134. 
Polysulphides, 213. 
Pomegranate, 326. 
Popp and Becker, 221, 222. 
Porgie oil, 447. 
Porter, 578. 
Potassium carbonate, 575. 

— ferrocyanide, 23. 

— hydrate, 181. 
Potential, equation to, 105. 
Potentilla, 326. 
Power-transmission, 542. 
Precipitation point of chrome 

liquors, 270. 
Preller, 460. 
Press-leaches, 394. 
Pricking, 156. 
Primary colours, 509. 
Prior, 475. 
Procter, 89, 148, 577, 582, 583, 656, 

665. 

— and Wilson, 268, 626. 
Protaceae, 303. 
Protamines, 126. 
Protea, 304. 
Protective colloids, 108. 
Protein colour reactions, 140 
Proteins, 125. 

— acid hydrolysis of, 134. 

— action of formaldehyde on, 144. 
of halogens on, 146. 

of nitrous acid on, 143. 

— classification of, 131. 

— coagulation by heat, 142. 

— constitution of, 129. 

— crystallisation, 141. 

hydrolysis, 126, 128, 132. 

— nitrogen in, 142. 

— precipitation, 139. 
■ — solubility of, 138. 

- — sulphur content, 142. 



Proteins, sulphur reaction, 141. 
Proteoses, 132. 
Protoplasm, 10, 11. 
Prussian blue, 479. 
Pseudo-solutions, 107. 
Pterocarpus, 326. 
Ptomaines, 19. 
Ptyalin, 17. 
Puering, 8, 201, 218. 

— fermentation in, 19. 
Pulleys, 543. 

" Pulling down," 201 

with acids, 206. 

Pullman, 183. 
Pulsometers, 552. 
Pumps, 551. 
Punica, 326. 
Putrefaction, 15, 19. 

— by land-filtration, 566. 

— of sewage, 564. 
Pyrogallol, 574 

— tans, 339. 
Pyrotan, 577. 
Pyrus, 326. 

Quandony, 304. 
Quebrachia, 306. 
Quebracho, 306. 
Quercitron oak, 299. 
Quercus, 289. 
Quinone, 573. 
tanning, 573. 

Rabinowitsch, 17. 

Radium, 84. 

Reaglar, 27, 185, 189. 

Recovered fats, 559. 

Reddening of flesh, 38. 

"Reds," 342, 372. 

Re-egging, 248. 

Refractive index of oils, 435. 

Reid, 483. 

Reimer, 60. 

Reinforced concrete, 548, 553. 

Rete malpighi, 50. 

Rhamnaceag, 326. 

Rhatany, 305. 

Rheedia, 335. 

Rhizophora, 322. 

Rhizophoracese, 322. 

Rhus, 307, 315. 

— coriaria, 313. 

— cotinus, 311. 

— metopium, 313. 

— succedanea, 452. 
Ricinoleic acid, 430. 
Riems, 460. 
Roans, 372. 

Robertson, T. Brailsford, 116, 155. 



INDEX 



685 



Rochelle salt, 268, 572. 

Rock salt, 36. 

Rohm, 190, 220, 222, 277. 

Rolling, 366. 

Rona, 625. 

Roofing, 553. 

Rosaceag, 326. 

Roscoe and Scudder, 227. 

Rose spirit, 481. 

Rosenthal, 223. 

Rosin, 456. 

— oils, 455. 

" Rounding," 198. 

Royles Ltd., 71. 

Rubiacese, 316. 

Rumex, 300. 

Rupe, 341. 

Russia leather oil, 287. 

■ — leather, 31. 

Sabal, 284. 
Sabatier, 432. 
Saccharomycetes, 15. 
Saccharomyces mycoderma, 13, 

19- 
Sachsse and Kormann, 143. 
Sal bark, 335. 
Salicaceae, 285. 
Salicylic acid, 21, 29. ■ 
Salix, 285, 286. 
Salomon, 228. 
Salt, common, 22. 

— earth, 39. 

hydrolysis, 1 01. 

— in alum tannages, 495. 

— saturated solution, 121. 
stains, 22, 36. 

— use of in tawing, 242. 
Salted hides, soaking of, 157. 
Salting, 33. 

— of " packer " hides, 34. 

— out, no. 

Sampling and analysis of tans, 

345- 

— extracts, 345, 346. 

— solid materials, 347. 

— tools, Kathreiner's, 346. 
Sand, Dr H., 104, 232. 
Santalacese, 304. 
Sapindaceae, 325. 
Saponification, 426. 

— cold, 427. 

value of oils, 433. 

Sappanwood, 329. 
Sassafras, oil of, 31. 
Saturated fatty acids, 429, 431. 

— solutions, 94. 
Satze, 364. 
Sawing mills, 386. 
Saxifrageee, 319. 



Schiff, 340. 
Schinopsis, 306. 
Schinus, 307. 
" Schlott's grains," 37. 
Schmeija mill, 38 
Schryver, S. B., 155, 146. 
Schultz, 229, 257. 

— Jackson, 160, 561. 
Scilla, 284. 
Scleroproteins, 131. 
Scorza rossa, 283. 
Scouring, 463. 
Screw-fans, 526. 
Scuth, 556. 

Seagrave-Bewington system, 530. 
Seal oil, 447. 

Seaside grape, 303. 
Sebaceous glaifds, 54, 61 . 

— layer, 61. 
Secondary colours, 509. 
Semichrome, 379. 
Semipermeable membranes, 95. 
Senna leaves, 329. 

Septa, 14. . 

Septic tank, 567. 

Serum-albumin, 131. 

Sesame oil, 444. 

Settling tanks, 564. 

Setting volume of gelatin, 585. 

Sewage, 563. 

— disposal, 538. 

filters, 566. 

Seymour-Hadwen, Dr, 44. 
Seymour-Jones, 26, 46, 49, 57, 60, 

223,236, 238, 576, 579, 656. 

— sterilisation process, 236. 
Shafting, 543 
Shark-liver oil, 446. 
Shaving, 463. 

— mills, 386. 
Shellac glaze, 494. 
Shorea, 335. 
Shrone blacks, 495. 
Siegfried, 133. 

Silent boiling jets, 400. 

Silicic acid in water, 80. 

Silver tree, 304 

Sites for tanneries, 538. 

Skraup, 143. 

" Slipping " of hides, 32, 157. 

" Smutting off," 498. 

Snobar bark, 273, 283. 

Snowbush, 326. 

Soaking and washing, 7. 

— of hides, 156. 
Soaks, putrid, 160. 
Soaps, 426. 

— metallic, 427. 
Soap test, 66, 428. 

Soda, caustic, for soaking, 162. 



686 PRINCIPLES OF LEATHER MANUFACTURE 



Soda in water, 79. 
Sodium bisulphate, 23, 204. 
• — carbonate deposits, 41. 
• — chloride, 22. 

— hydrate, 181. 

— hypochlorite, 575. 

— perborate, 37. 

— sulphate, 23. 
in salt-earth, 41. 

— sulphide, 185. 

for soaking, 162. 

Sod oil, 448, 459. 

Sole leather, American finish, 367. 

drying, 365. 

finishing, 365. 

oiling, 365. 

tanning, 355. 

Solid extracts, 411. 
Sols, 107. 

Solubilising extracts, 405. 
Soluble phenyl, 28. 
Solution, 92. 

— of solids, 94. 

— pressure, 92, 96. 
Solutions obey gas-laws, 96. 
Solvents, immiscible, 93. 
Sommerhoff, 577. 

Sorbus, 326. 
Sorensen, 104, 661. 

— exponential scale, 106. 
South American hides, 42. 
Specific gravity of oils, 434. 
Spent tan, 560. 

Sperm oil, 450. 

Spetches, 556. 

Splitting, 62, 463. 

Sprinkler leaches, 403. 

Sprinklers, 540. 

" Spueing," 20, 428, 430, 469. 

Squill, 284. 

Staining, 507. 

Staking, 244. 

" Staling " of sheepskins, 33. 

Starch, 474. 

— paste in drum-tannage, 573. 
Statice, 305. 

Steam-jet water raisers, 552. 
Steampipes, arrangement, 525. 

— expansion, 532. 
Steam traps, 533. 
Steapsin, 223. 
Stearine glaze, 494. 
" Stearines," 437. 
Stefan, 88. 
Step-grates, 561. 
Sterilisation, 17. 
Sternolessis, 325. 

Stiasny, 114, 182, 190, 209, 238, 
260, 270, 272, 343, 572, 596, 
657, 669. 



Stiasny and Das, 259. 

Stippen, 157. 

Stirring by compressed air, 554. 

Stocks, 163. 

Strainers, 552. 

Straits oil, 447. 

Stratum lucidum, 51. 

— mucosum, 51. 
Striking, 365. 

Stripping chrome leather, 379, 571, 

572- 
Strong acids and bases, 98. 
Structure of skin, 48. 
Stryphnodendron, 335. 
Sturtevant system, 530, 535. 
Sudoriferous glands, 49, 55. 
Sugar bush, 304. 

— action on gelatin, 115. 
Sulphate of alumina, 241. 
Sulphide of sodium, 185. 
Sulphides in liming, 185. 
Sulphonated castor oil, 441. 

— fish oils, 471, 473. 

— oils, 274. 
Sulphonic acids, 29. 
Sulphur-bacteria, 79. 

— dioxide, 24. 

— in chrome leather, 272. 

— reaction of proteins, 141. 
Sulphuric acid in water, 80. 
Sulphurous acid, 24, 161. 
Sumach, 307. 

— adulterants, 309. 

— galls, 315. 
Sumaching, 374, 503. 
Sumachs, American, 310. 
Supersaturated solutions, 94. 
Surface energy, 90. 

— films, 90, 91. 

— tension, 88, 465. 

of solutions, 89. 

Suspenders, 356. 
Suspension-colloids, 107. 
Suspension of leather in drying, 528. 
Swan, 256. 

Sweat-glands, 49, 55. 
" Sweating," 19, 166. 

— of sheepskins, 33. 
Sweat-pit, i56. 

Swedish glove leather, 377. 
Sweet fern, 286. 
Swelling of gelatin, 114, 582. 
Syntans, 343, 572. 

Table grease, 466. 
Takout galls, 319. 
Tallow, 436. 

— from grease, 559. 
Tamariscinise, 319. 
Tamarix, 319. 



INDEX 



687 



Tamarix Africana, 312. 
Tamwood, 305. 
Tan-burning, 560. 
Tanekahi bark, 284. 
Tan-furnace, 561. 
Tanghadi bark, 329. 
Tank- waste, 188. 
Tannage and dyeing, 490. 
Tannery waste to sewers, 565. 
Tannin analysis, 579. 

— colour-lakes, 490. 

— distribution, 278. 
Tanning, 8. 

— fermentation in, 19. 

— object of, 7. 
Tannins, charges of, 118. 

— chemistry of, 337. 
Tan-pressing, 563. 
Tar-brands, 43. 
Tari pods, 327. 
Tarsekahi bark, 284. 
Tartar emetic, 498. 
Tawing, 9, 240. 

— paste, 247. 
Teed, 67. 
Teel oil, 444. 
Temperature, absolute zero, 85. 

— of dyeing, 501. 

resistance of skins, 211. 

Tengah bark, 323. 

Terminalia, 320. 

Terra Japonica, 316. 

Tertiary colours, 509. 

Thann leaves, 321. 

Thawai, 336. 

Theories of tanning, 578. 

Thermophilic bacteria, 17. 

Thiosulphate, redaction with, 

259- 
Thiothrix, 79. 
Thomas, 656. 
Thompson, 656. 

— and Suzuki, 208, 672. 
Thorp, 479. 

Thrift, 305. 
Thuau, 323. 
Tick- marks, 43. 
Tilco bark, 324. 
Tinned meats, 17. 
Titanium salts, 498. 
Tjamara laut, 285. 
" Topping," 499. 
Tormentilla, 326. 
Torulo, 13. 

— in salt-stains, 37. 
Towai or Tawheri bark, 319. 
Towse, W., 39. 

Tragosol, 573. 

Transmission of power, 542. 
Tray-dyeing (Continental), 501. 



Tray-dyeing (English), 499. 

Triformol, 31. 

Trimble, 282. 

Tri-oxymethylene, 31. 

Troy weight, 581. 

True acidity and alkalinity, loi. 

Trypsin, 17, 64, 137, 219, 220. 

Tryptic enzyme for unhairing, 190. 

Tsuga, 282. 

Tubular boilers unsuitable for tan, 

562. 
Tugwar or tulwah, 335. 
Tunnel drier, 558. 
Turkey oak, 292. 

red oil, 441. 

Turnbull, 573. 

Turret drier, 530. 

Turwad bark, 329. 

Two-bath chrome process, 257. 

Tyndall effect, no, 113. 

Tyrosine, 127. 

Ulmo, 336. 
Ultra-filtration, 11 1. 

microscope, 107, 108. 

violet light, 483, 484. 

Umschlagen, 215. 
Uncaria, 318. 
Unhairing, 191. 

— by trypsin, 138. 

— machines, 192. 

— processes, 64. 

— with stocks, 192. 

— with tryptic enzyme, 190. 

— with wash-wheel, 192. 
Unsaturated fatty acids, 428, 432. 
Unwooling by " painting," 33. 
Use of extracts, 411. 

Vaccinias, 319. 

Vaccinium, 319. 

Vacuoles, 12. 

Valdivia leather, 303. 

Valonia for dressing leather, 364. 

— oak, 295. 

Van Bemmelen, 112, 588, 624. 
Van der Waals, 86, 89. 
Van Slyke, 144. 
Van Tieghem, 279. 
Vaney, Prof. C, 43. 
Vapour-pressure, 86, 512. 

of water, 517. 

Varrons, 43. 

Vaseline, 454. 

Vateria, 335. 

Vatting, 374. 

Vaughn machine, 195. 

Vegetable tanning process, 355. 

Veley, 661. 

Venetian sumach, 315. 



688 PRINCIPLES OF LEATHER MANUFACTURE 



Ventilation, 520. 
Versenke, 364. 
Vignon, 77. 
Villon, 481. 
Viscosimeter, iii, 113. 
Viscosity, iii. 
Vissoko, stream at, 82. 
Volatile oils, 452. 
Voluntary muscle, 63. 
Von Hohnel, 278. 

Von Schroeder, 176, 180, 186, 616, 
625. 

■ and Paessler, 147. 

Von Weimarn, 141. 

Waagenboom, 304. 
Walpole, 103, 661. 
Warble flies, 43. 
Wash -wheel, 158. 
Waste liquors, 563. 

— products, 555. 

fatal to ferment-organisms, 18. 

Water, cost of softening, 78. 

— for boilers, 77. 
■ — hammer, 532. 

— impurities of, 66. 

— supply, 539. 
Waterproofing, 427. 

— leather, 469. 
Wattle bark, 332. 
Waxes, 450. 

Weak acids and bases, 98. 

— grain, 156. 
Weigert's stain, 63. 
Weimania, 319. 

Wet and dry bulb thermometer, 

518. 
Whale oil, 446. 
White bark, 334. 

— tan, 327. 
Wild almond, 304. 
Williams' crusher, 384. 



Willow, 285. 

— bark, 377. 

Wilson, 182, 223, 261, 351, 363, 579, 
582,583.656. 

— E., 531- 

— W. H., 583. 

• — and Kern, 77, 80. 

— fleshing machine, 199. 
Windbores, 552. 
Wintergreen, oil of, 31, 287. 
Witt, 489. 

Wood, 17. 

Wood, J. T., 29, 104, 152, 200, 201, 

203, 208, 214, 219, 222, 229, 

231,449,461,578. 

— and Wilcox, 218. 

— Sand and Law, 660. 
Wood dyes, 505. 
Wooden pits, 548. 
Wool-fat, 439. 

sorter's disease, 17. 

Wringing, 459. 

Wyoming, salt deposits of, 41. 

Xanthoproteic reaction, 140. 
Xylia, 335. 

Yaryan evaporator, 406, 407, 408. 
Yeast-cells, 12 
Yorkshire flagstones, 548. 

— grease, 439. 

Youl and Griffith, 364. 

Zacharias, 489. 

Zeolites, 73. 

Zinc chloride, 27, 37. 

— sulphate, 27, 209. 
Zizyphus, 326. 
Zollikoffer, 210. 
Zsigmondy, 107. 
Zymases, 15. 

Zymases of puer-liquor, 1 7. 



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